WHO warns of 'second peak' in COVID-19 infections

A pandemic of the novel coronavirus has now killed more than 345,000 people worldwide.

Over 5.4 million people across the globe have been diagnosed with COVID-19, the disease caused by the new respiratory virus, according to data compiled by the Center for Systems Science and Engineering at Johns Hopkins University. The actual numbers are believed to be much higher due to testing shortages, many unreported cases and suspicions that some governments are hiding the scope of their nations' outbreaks.

Since the first cases were detected in China in December, the United States has become the worst-affected country, with more than 1.6 million diagnosed cases and at least 97,724 deaths.

Tune into ABC at 1 p.m. ET and ABC News Live at 4 p.m. ET every weekday for special coverage of the novel coronavirus with the full ABC News team, including the latest news, context and analysis.

Monday's biggest developments:

Here's how the news developed on Monday. All times Eastern. Please refresh this page for updates.

11:42 p.m.: Brazilians board last-minute flights to US ahead of travel ban

Brazilians scrambled to board last-minute flights to the United States Monday ahead of President Donald Trump's new travel restrictions.

Groups of passengers were seen at Sao Paulo's Guarulhos International Airport preparing to board a United flight to Houston after Trump announced the upcoming restrictions on travel due to the rising number of COVID-19 cases in Brazil.

Brazil now has 374,898 confirmed cases of the virus, according to data compiled by Johns Hopkins University, making it the country with the second most coronavirus cases worldwide after the U.S.

8:17 p.m.: St. Louis County issues travel advisory after residents gather at lake party

People in Missouri's St. Louis County failed to follow social distancing rules over Memorial Day weekend, with large crowds gathering at Lake of the Ozarks for a party. This comes as businesses in the area begin to reopen.

County Executive Dr. Sam Page asked the Department of Public Health Monday to issue a travel advisory as some residents return to work.

"This reckless behavior endangers countless people and risks setting us back substantially from the progress we have made in slowing the spread of CVOD-19," Page stated in a St. Louis County press release. "I encourage everyone to follow the Department of Public Health advisory to determine a safe path forward in the workplace."

The DPH urged those who ignored protective practices to self-quarantine for 14 days or until testing negative for COVID-19, according to the press release.

Current DPH guidance recommends that employers screen employees for health risks before allowing them to return to the workplace. Employers should also consider asking their employees about their recent travels and social distancing practices.

5:53 p.m.: WHO warns of 'second peak' in COVID-19 infections

The World Health Organization's emergencies director, Dr. Mike Ryan, told reporters Monday that countries with declining cases still face danger or an "immediate second peak" if they ease restrictions too soon.

"The disease can jump up at any time. We cannot make assumptions that just because the disease is on the way down now it is going to keep going down and we are get a number of months to get ready for a second wave. We may get a second peak in this wave," he said, adding that countries in Europe and North America should "continue to put in place the public health and social measures, the surveillance measures, the testing measures and a comprehensive strategy to ensure that we continue on a downwards trajectory and we don't have an immediate second peak."

4:23 p.m.: California to allow religious services, reopens in-person retail

The California Department of Public Health announced it is rolling back its restrictions on religious services and in-person retail.

Places of worship can hold services and funerals that limit attendance to 25% of a building's capacity, or up to 100 attendees, whichever is lower, if it gets approval from their local health departments. The places of worship must also follow other guidelines listed on the health department's website.

Store owners would also need approval from their local health departments before they can allow for in-person customers and they must also follow strict distancing rules. Personal services such as hair salons, nail salons and barbershops will remain closed, according to the new guidelines.

4:05 p.m.: UK to reopen stores starting June 1

British Prime Minister Boris Johnson said starting June 1, non-essential retail can begin to reopen.

The first group of stores that will be permitted will be outdoor markets and car show rooms. After June 15, all other non-essential retail, provided strict hygiene and social distancing measures are in place, will be permitted.

"These are careful but deliberate steps on the road to rebuilding our country -- and we can only take these thanks to what we have so far achieved together," Johnson said at a news conference.

12:50 p.m.: WHO suspends hydroxychloroquine study

The World Health Organization said it would pause its study on the anti-malaria drug hydroxychloroquine as part of its ongoing research into COVID-19 treatments.

The move comes after a study was published in medical journal The Lancet that found coronavirus patients who took the drug were more likely to die or develop irregular heart rhythms. WHO said its executive group of the Solidarity Trial is reviewing data on its research into the drug while the pause goes into effect.

"The other arms of the trial are continuing," Dr. Samba Sow, director general of the Center for Vaccine Development in Mali, said at a news conference. "This concern relates to the use of hydroxychloroquine and chloraquine in COVID-19."

12:05 p.m.: New York to provide death benefits to all public front-line workers

New York Gov. Andrew Cuomo announced that the state will provide death benefits to all workers on the front line who have died during the pandemic.

That includes police officers, EMTs, firefighters, hospital workers and transit employees.

"They showed up because I asked them to show up," Cuomo said at his daily briefing. "I have such respect and esteem for what they did … and I want to make sure we repay that."

Cuomo called on the federal government to approve a similar move for front-line workers across the nation.

He said the state's efforts to curb the number of cases continues to show promise. There were 96 COVID-19 deaths in the state within the last 24 hours, according to the governor's office.

That's a drop of 13 deaths recorded the previous day. The total number of hospitalizations was 4,348 on Sunday, which represented a 45 drop from Saturday, Cuomo's office said.

11:33 a.m.: Putin's spokesman reportedly discharged from hospital after coronavirus bout

Russian President Vladimir Putin's longtime press secretary, Dmitry Peskov, has been discharged from a hospital in Moscow after recovering from COVID-19, according to Russian media reports.

"Yes, that's true," Peskov told the independent Interfax news agency on Monday. He also confirmed that he must stay home for two weeks upon leaving the hospital.

On May 12, Peskov was quoted telling Russian state-run news agency RIA Novosti that he was hospitalized after testing positive for the novel coronavirus.

11:02 a.m.: Trump administration to buy 100 million swabs but leaves testing responsibility to states

The U.S. government plans to acquire 100 million swabs and distribute them to states to help expand the country's capacity to test for the novel coronavirus, according to a new report submitted to Congress and obtained by ABC News.

While the Trump administration was required to submit a national testing plan to Congress under a law passed last month, the 81-page report submitted Sunday doesn't appear to be a new strategy but rather a continuation of the White House's existing posture: that states should take the lead in expanding their own testing capacity. The Washington Post was first to report on the contents of the document.

"With support from the Federal government to ensure States are meeting goals, the State plans for testing will advance the safe reopening of America," the Trump administration wrote in the report's conclusion.

The U.S. government still projects the country will be able to produce 40 million to 50 million COVID-19 tests a month by September, and it recommends that every state should be able to test 2% of its population in May and June.

10:40 a.m.: Spain to end mandatory quarantine for international travelers in July

Spain will lift a two-week mandatory quarantine for all travelers arriving from abroad starting July 1, the government announced Monday.

"The worst is behind us," Spanish Foreign Minister Arancha Gonzalez Laya wrote on Twitter, adding that next month officials will "gradually" open to international tourists, lift the quarantine measures and "ensure the highest standards of health safety."

Spain is one of the worst-affected countries in the coronavirus pandemic, with more than 235,000 diagnosed cases of COVID-19 and at least 28,752 deaths, according to a count kept by Johns Hopkins University.

9:54 a.m.: Wuhan conducts over 6.5 million coronavirus tests in 10 days

The Chinese city of Wuhan, ground zero of the coronavirus pandemic, has conducted more than 6.5 million nucleic acid tests for the novel coronavirus over a 10-day period, according to state media reports.

Earlier this month, the Wuhan Municipal Health Commission launched a citywide campaign to test the entire population of 11 million residents for COVID-19 in an effort to search for asymptomatic carriers of the virus after a cluster of new cases emerged for the first time since the city had lifted its strict lockdown on April 8.

Although recommended, participation in the testing campaign is voluntary. Residents who were previously tested do not need to take part. It is not recommended to test children under the age of 6, according to the Wuhan Municipal Health Commission's website.

No new symptomatic cases of COVID-19 have been reported since the campaign began; however, dozens of people without symptoms have tested positive for the virus, according to daily reports published by the Wuhan Municipal Health Commission. The local health authority is asking anyone who hasn't been tested yet to come forward by the end of Tuesday.

Prior to the campaign, the city had completed over 3 million nucleic acid tests for COVID-19, according to the official state-run Xinhua News Agency.

9:06 a.m.: Austria completes 1st coronavirus lung transplant in Europe

A team of doctors in Austria's capital have successfully conducted a lung transplant on a coronavirus patient -- the first to be done in Europe.

The Medical University of Vienna announced the news in a statement Monday, saying its surgeons had carried out the procedure at Vienna General Hospital last week on a 45-year-old woman who had developed "severe respiratory failure" due to COVID-19. The team replaced the patient's lungs, "which had been damaged beyond repair," with those from a donor in what the university described as "an otherwise hopeless situation."

"We are very satisfied with the patient's condition, given the extremely difficult initial circumstances," Dr. Walter Klepetko, head of surgery at the clinic, said in a statement released by the university. "Only a few days after the procedure, the patient is well on the way to recovery."

The patient, from the southern state of Carinthia, had contracted the novel coronavirus about eight weeks ago. As her condition deteriorated, artificial ventilation was no longer possible and a circulation pump was the only thing keeping her alive, according to the university.

The woman was transported to Vienna General Hospital where she was admitted into the intensive care unit. A diagnostic test showed that particles of the novel coronavirus were still present in her system, but a negative viral culture subsequently confirmed she was no longer infectious. With no chance of the woman's lungs recovering but her other organs still functioning, the team of doctors decided to perform "an urgent and highly complex" transplant. The donor lungs were flown in from a neighboring country.

"The transplant itself took place under extremely difficult circumstances, since the patient did not have an adequate blood platelet count and, since antibodies were also present, these first had to be removed by means of immune aphaeresis to prevent her from rejecting the organ," Klepetko said. "Even transportation of the lungs and preparation for the operation took place under difficult conditions, especially because of the necessary COVID-19 logistics and the associated protective measures, which all had to be observed. In cases such of this, the key to success is smooth and effective collaboration between the various professional groups, such as anaesthesia, surgery, intensive care medicine, infectiology and many others."

8:38 a.m.: Montenegro declares itself virus-free

Montenegro has become the second country in Europe to declare itself free of the novel coronavirus.

Montenegro's Institute for Public Health announced via Twitter on Sunday that the tiny Balkan nation currently has no active cases of COVID-19. The news comes 68 days after Montenegro recorded its first case of the disease.

"As of yesterday, our laboratories have completed analysis of 140 respiratory samples, among which there were no new cases of infection," the institute tweeted. "At the same time, the last active case met the criteria for full recovery."

Montenegro, which is home to some 631,000 people, has reported 324 cases of COVID-19 with nine deaths, according to a count kept by Johns Hopkins University.

Earlier this month, Slovenia became the first European nation declare itself coronavirus-free.

7:10 a.m.: South Korea to require masks on transit, flights

People must wear face masks when using public transportation and taxis in South Korea starting Tuesday, as part of efforts to slow the spread of the novel coronavirus while public activities are increased.

South Korean health ministry official Yoon Taeho said masks will also be required on all domestic and international flights from Wednesday. Meanwhile, starting in June, owners of "high-risk" facilities such as bars, gyms and concert halls will be required to register customers at the door using smartphone QR codes to aid contact tracing when infections occur.

"Until treatments and vaccines are developed," we will never know when the COVID-19 crisis could end, and until then we will have to learn how to live with COVID-19," Yoon told reporters Monday.

South Korea once had the largest novel coronavirus outbreak outside China, where the virus first emerged, but appears to have brought it largely under control with an extensive "trace, test and treat" strategy. A total of 11,206 people in the country have been diagnosed with COVID-19, of which 10,226 have recovered and 267 have died, according to South Korea's Centers for Disease Control and Prevention.

Earlier this month, South Korea returned to a sense of normalcy as the nation eased its strict social-distancing measures that were put in place to curb the spread of the virus. The number of new cases reported in the country has generally stayed low, but health authorities remain wary of cluster infections and imported cases. More than 200 cases were recently linked to reopened bars, nightclubs and other entertainment venues in Seoul, the densely populated capital.

6:23 a.m.: Russia surpasses 350,000 coronavirus cases

Russia reported 8,946 new cases of COVID-19 on Monday, bringing the country's tally to 353,427.

Russia's coronavirus response headquarters also recorded 92 coronavirus-related deaths over the past 24 hours, placing the nationwide toll at 3,633, which is considerably lower than many other countries hit hard by the pandemic.

The daily rise in deaths has declined by a third after a record high of 153 reported on Sunday. Meanwhile, the latest daily caseload is down from a peak of 11,656 new infections reported on May 11, during which Russia registered over 10,000 new cases per day over a 12-day period. Since then, the daily number of new infections has hovered around 9,000 per day.

Over the weekend, Brazil surpassed Russia as the country with the second-highest number of diagnosed COVID-19 cases in the world, behind the United States, according to a count kept by Johns Hopkins University.

5:31 a.m.: Japan lifts state of emergency in Tokyo and other remaining areas

Japanese Prime Minister Shinzo Abe lifted the state of emergency in Tokyo and four other remaining areas on Monday, bringing an end to the restrictions nationwide.

A government-commissioned panel of experts approved the move in the prefectures of Tokyo, Saitama, Kanagawa, Hokkaido and Chiba, all of which had remained under the emergency declaration after it was lifted for most of Japan earlier this month.

The prime minister first declared a monthlong state of emergency for Tokyo and six other prefectures on April 7, as Japan reported a surge in COVID-19 cases. He later expanded the declaration to cover the entire country and last until May 31. Under the order, prefectural governors asked residents to stay home and for some businesses to temporarily close, but public cooperation was voluntary. There were no penalties for failure to comply.

On May 13, Abe ended the state of emergency in 39 of the country's 47 prefectures. The decree remained in place for urban regions, including the capital, Tokyo, and the large port city of Osaka.

Abe lifted the state of emergency in Osaka as well as Kyoto and Hyogo on May 21, following a drop in the number of new cases reported each day. Chiba, Hokkaido, Kanagawa, Saitama and Tokyo all remained under restrictions until now.

The prime minister has credited the recent decline in new infections to the efforts of residents staying at home and practicing social distancing. He warned, however, that the state of emergency may have to be reimposed if infections increase.

More than 16,500 people in Japan have been diagnosed with COVID-19 and at least 820 have died, according to a count kept by Johns Hopkins University.

5:18 a.m.: US reports over 20,600 new coronavirus cases

More than 20,600 people in the United States were diagnosed with COVID-19 on Sunday, according to a count kept by Johns Hopkins University.

The country also reported over 600 new deaths from the disease on the same day.

The United States is, by far, the hardest-hit country in the coronavirus pandemic.

What to know about coronavirus:

3:45 a.m.: Houston receives hundreds of social distancing complaints over the weekend

Authorities in Houston said they have received hundreds of social distancing complaints over Memorial Day weekend amid the coronavirus pandemic.

Texas Gov. Greg Abbott's executive order allows bars to reopen at 25% of their normal capacity and restaurants at 50%. However, Houston Fire Chief Samuel Pena said via Twitter on Sunday afternoon that his department had addressed around 300 complaints of violation to the governor's rules since Friday, adding that "admittance beyond approved capacity will cause events to be stopped until condition is corrected."

Houston Mayor Sylvester Turner announced 115 new confirmed cases of COVID-19 with one additional death in the city on Sunday, bringing the total to 6,640 cases with 126 deaths.

Turner also said he saw photos and videos of people flouting the social distancing rules at crowded bars and packed pool parties over the holiday weekend. The mayor pleaded with businesses and customers to "be responsible," noting that their behavior puts first responders at risk, too.

"We don't want to be heavy handed," Turner said at a press conference Sunday. "If you work with us, nobody gets closed down."

ABC News' Clark Bentson, Marilyn Heck, Alina Lobzina and Ben Siegel contributed to this report.

Brazilians scrambled to board last-minute flights to the United States Monday ahead of President Donald Trump's new travel restrictions.

Groups of passengers were seen at Sao Paulo's Guarulhos International Airport preparing to board a United flight to Houston after Trump announced the upcoming restrictions on travel due to the rising number of COVID-19 cases in Brazil.

Brazil now has 374,898 confirmed cases of the virus, according to data compiled by Johns Hopkins University, making it the country with the second most coronavirus cases worldwide after the U.S.

8:17 p.m.: St. Louis County issues travel advisory after residents gather at lake party

People in Missouri's St. Louis County failed to follow social distancing rules over Memorial Day weekend, with large crowds gathering at Lake of the Ozarks for a party. This comes as businesses in the area begin to reopen.

County Executive Dr. Sam Page asked the Department of Public Health Monday to issue a travel advisory as some residents return to work.

"This reckless behavior endangers countless people and risks setting us back substantially from the progress we have made in slowing the spread of CVOD-19," Page stated in a St. Louis County press release. "I encourage everyone to follow the Department of Public Health advisory to determine a safe path forward in the workplace."

The DPH urged those who ignored protective practices to self-quarantine for 14 days or until testing negative for COVID-19, according to the press release.

Current DPH guidance recommends that employers screen employees for health risks before allowing them to return to the workplace. Employers should also consider asking their employees about their recent travels and social distancing practices.

5:53 p.m.: WHO warns of 'second peak' in COVID-19 infections

The World Health Organization's emergencies director, Dr. Mike Ryan, told reporters Monday that countries with declining cases still face danger or an "immediate second peak" if they ease restrictions too soon.

"The disease can jump up at any time. We cannot make assumptions that just because the disease is on the way down now it is going to keep going down and we are get a number of months to get ready for a second wave. We may get a second peak in this wave," he said, adding that countries in Europe and North America should "continue to put in place the public health and social measures, the surveillance measures, the testing measures and a comprehensive strategy to ensure that we continue on a downwards trajectory and we don't have an immediate second peak."

4:23 p.m.: California to allow religious services, reopens in-person retail

The California Department of Public Health announced it is rolling back its restrictions on religious services and in-person retail.

Places of worship can hold services and funerals that limit attendance to 25% of a building's capacity, or up to 100 attendees, whichever is lower, if it gets approval from their local health departments. The places of worship must also follow other guidelines listed on the health department's website.

Store owners would also need approval from their local health departments before they can allow for in-person customers and they must also follow strict distancing rules. Personal services such as hair salons, nail salons and barbershops will remain closed, according to the new guidelines.

4:05 p.m.: UK to reopen stores starting June 1

British Prime Minister Boris Johnson said starting June 1, non-essential retail can begin to reopen.

The first group of stores that will be permitted will be outdoor markets and car show rooms. After June 15, all other non-essential retail, provided strict hygiene and social distancing measures are in place, will be permitted.

"These are careful but deliberate steps on the road to rebuilding our country -- and we can only take these thanks to what we have so far achieved together," Johnson said at a news conference.

12:50 p.m.: WHO suspends hydroxychloroquine study

The World Health Organization said it would pause its study on the anti-malaria drug hydroxychloroquine as part of its ongoing research into COVID-19 treatments.

The move comes after a study was published in medical journal The Lancet that found coronavirus patients who took the drug were more likely to die or develop irregular heart rhythms. WHO said its executive group of the Solidarity Trial is reviewing data on its research into the drug while the pause goes into effect.

"The other arms of the trial are continuing," Dr. Samba Sow, director general of the Center for Vaccine Development in Mali, said at a news conference. "This concern relates to the use of hydroxychloroquine and chloraquine in COVID-19."

12:05 p.m.: New York to provide death benefits to all public front-line workers

New York Gov. Andrew Cuomo announced that the state will provide death benefits to all workers on the front line who have died during the pandemic.

That includes police officers, EMTs, firefighters, hospital workers and transit employees.

"They showed up because I asked them to show up," Cuomo said at his daily briefing. "I have such respect and esteem for what they did … and I want to make sure we repay that."

Cuomo called on the federal government to approve a similar move for front-line workers across the nation.

He said the state's efforts to curb the number of cases continues to show promise. There were 96 COVID-19 deaths in the state within the last 24 hours, according to the governor's office.

That's a drop of 13 deaths recorded the previous day. The total number of hospitalizations was 4,348 on Sunday, which represented a 45 drop from Saturday, Cuomo's office said.

11:33 a.m.: Putin's spokesman reportedly discharged from hospital after coronavirus bout

Russian President Vladimir Putin's longtime press secretary, Dmitry Peskov, has been discharged from a hospital in Moscow after recovering from COVID-19, according to Russian media reports.

"Yes, that's true," Peskov told the independent Interfax news agency on Monday. He also confirmed that he must stay home for two weeks upon leaving the hospital.

On May 12, Peskov was quoted telling Russian state-run news agency RIA Novosti that he was hospitalized after testing positive for the novel coronavirus.

11:02 a.m.: Trump administration to buy 100 million swabs but leaves testing responsibility to states

The U.S. government plans to acquire 100 million swabs and distribute them to states to help expand the country's capacity to test for the novel coronavirus, according to a new report submitted to Congress and obtained by ABC News.

While the Trump administration was required to submit a national testing plan to Congress under a law passed last month, the 81-page report submitted Sunday doesn't appear to be a new strategy but rather a continuation of the White House's existing posture: that states should take the lead in expanding their own testing capacity. The Washington Post was first to report on the contents of the document.

"With support from the Federal government to ensure States are meeting goals, the State plans for testing will advance the safe reopening of America," the Trump administration wrote in the report's conclusion.

The U.S. government still projects the country will be able to produce 40 million to 50 million COVID-19 tests a month by September, and it recommends that every state should be able to test 2% of its population in May and June.

10:40 a.m.: Spain to end mandatory quarantine for international travelers in July

Spain will lift a two-week mandatory quarantine for all travelers arriving from abroad starting July 1, the government announced Monday.

"The worst is behind us," Spanish Foreign Minister Arancha Gonzalez Laya wrote on Twitter, adding that next month officials will "gradually" open to international tourists, lift the quarantine measures and "ensure the highest standards of health safety."

Spain is one of the worst-affected countries in the coronavirus pandemic, with more than 235,000 diagnosed cases of COVID-19 and at least 28,752 deaths, according to a count kept by Johns Hopkins University.

9:54 a.m.: Wuhan conducts over 6.5 million coronavirus tests in 10 days

The Chinese city of Wuhan, ground zero of the coronavirus pandemic, has conducted more than 6.5 million nucleic acid tests for the novel coronavirus over a 10-day period, according to state media reports.

Earlier this month, the Wuhan Municipal Health Commission launched a citywide campaign to test the entire population of 11 million residents for COVID-19 in an effort to search for asymptomatic carriers of the virus after a cluster of new cases emerged for the first time since the city had lifted its strict lockdown on April 8.

Although recommended, participation in the testing campaign is voluntary. Residents who were previously tested do not need to take part. It is not recommended to test children under the age of 6, according to the Wuhan Municipal Health Commission's website.

No new symptomatic cases of COVID-19 have been reported since the campaign began; however, dozens of people without symptoms have tested positive for the virus, according to daily reports published by the Wuhan Municipal Health Commission. The local health authority is asking anyone who hasn't been tested yet to come forward by the end of Tuesday.

Prior to the campaign, the city had completed over 3 million nucleic acid tests for COVID-19, according to the official state-run Xinhua News Agency.

9:06 a.m.: Austria completes 1st coronavirus lung transplant in Europe

A team of doctors in Austria's capital have successfully conducted a lung transplant on a coronavirus patient -- the first to be done in Europe.

The Medical University of Vienna announced the news in a statement Monday, saying its surgeons had carried out the procedure at Vienna General Hospital last week on a 45-year-old woman who had developed "severe respiratory failure" due to COVID-19. The team replaced the patient's lungs, "which had been damaged beyond repair," with those from a donor in what the university described as "an otherwise hopeless situation."

"We are very satisfied with the patient's condition, given the extremely difficult initial circumstances," Dr. Walter Klepetko, head of surgery at the clinic, said in a statement released by the university. "Only a few days after the procedure, the patient is well on the way to recovery."

The patient, from the southern state of Carinthia, had contracted the novel coronavirus about eight weeks ago. As her condition deteriorated, artificial ventilation was no longer possible and a circulation pump was the only thing keeping her alive, according to the university.

The woman was transported to Vienna General Hospital where she was admitted into the intensive care unit. A diagnostic test showed that particles of the novel coronavirus were still present in her system, but a negative viral culture subsequently confirmed she was no longer infectious. With no chance of the woman's lungs recovering but her other organs still functioning, the team of doctors decided to perform "an urgent and highly complex" transplant. The donor lungs were flown in from a neighboring country.

"The transplant itself took place under extremely difficult circumstances, since the patient did not have an adequate blood platelet count and, since antibodies were also present, these first had to be removed by means of immune aphaeresis to prevent her from rejecting the organ," Klepetko said. "Even transportation of the lungs and preparation for the operation took place under difficult conditions, especially because of the necessary COVID-19 logistics and the associated protective measures, which all had to be observed. In cases such of this, the key to success is smooth and effective collaboration between the various professional groups, such as anaesthesia, surgery, intensive care medicine, infectiology and many others."

8:38 a.m.: Montenegro declares itself virus-free

Montenegro has become the second country in Europe to declare itself free of the novel coronavirus.

Montenegro's Institute for Public Health announced via Twitter on Sunday that the tiny Balkan nation currently has no active cases of COVID-19. The news comes 68 days after Montenegro recorded its first case of the disease.

"As of yesterday, our laboratories have completed analysis of 140 respiratory samples, among which there were no new cases of infection," the institute tweeted. "At the same time, the last active case met the criteria for full recovery."

Montenegro, which is home to some 631,000 people, has reported 324 cases of COVID-19 with nine deaths, according to a count kept by Johns Hopkins University.

Earlier this month, Slovenia became the first European nation declare itself coronavirus-free.

7:10 a.m.: South Korea to require masks on transit, flights

People must wear face masks when using public transportation and taxis in South Korea starting Tuesday, as part of efforts to slow the spread of the novel coronavirus while public activities are increased.

South Korean health ministry official Yoon Taeho said masks will also be required on all domestic and international flights from Wednesday. Meanwhile, starting in June, owners of "high-risk" facilities such as bars, gyms and concert halls will be required to register customers at the door using smartphone QR codes to aid contact tracing when infections occur.

"Until treatments and vaccines are developed," we will never know when the COVID-19 crisis could end, and until then we will have to learn how to live with COVID-19," Yoon told reporters Monday.

South Korea once had the largest novel coronavirus outbreak outside China, where the virus first emerged, but appears to have brought it largely under control with an extensive "trace, test and treat" strategy. A total of 11,206 people in the country have been diagnosed with COVID-19, of which 10,226 have recovered and 267 have died, according to South Korea's Centers for Disease Control and Prevention.

Earlier this month, South Korea returned to a sense of normalcy as the nation eased its strict social-distancing measures that were put in place to curb the spread of the virus. The number of new cases reported in the country has generally stayed low, but health authorities remain wary of cluster infections and imported cases. More than 200 cases were recently linked to reopened bars, nightclubs and other entertainment venues in Seoul, the densely populated capital.

6:23 a.m.: Russia surpasses 350,000 coronavirus cases

Russia reported 8,946 new cases of COVID-19 on Monday, bringing the country's tally to 353,427.

Russia's coronavirus response headquarters also recorded 92 coronavirus-related deaths over the past 24 hours, placing the nationwide toll at 3,633, which is considerably lower than many other countries hit hard by the pandemic.

The daily rise in deaths has declined by a third after a record high of 153 reported on Sunday. Meanwhile, the latest daily caseload is down from a peak of 11,656 new infections reported on May 11, during which Russia registered over 10,000 new cases per day over a 12-day period. Since then, the daily number of new infections has hovered around 9,000 per day.

Over the weekend, Brazil surpassed Russia as the country with the second-highest number of diagnosed COVID-19 cases in the world, behind the United States, according to a count kept by Johns Hopkins University.

5:31 a.m.: Japan lifts state of emergency in Tokyo and other remaining areas

Japanese Prime Minister Shinzo Abe lifted the state of emergency in Tokyo and four other remaining areas on Monday, bringing an end to the restrictions nationwide.

A government-commissioned panel of experts approved the move in the prefectures of Tokyo, Saitama, Kanagawa, Hokkaido and Chiba, all of which had remained under the emergency declaration after it was lifted for most of Japan earlier this month.

The prime minister first declared a monthlong state of emergency for Tokyo and six other prefectures on April 7, as Japan reported a surge in COVID-19 cases. He later expanded the declaration to cover the entire country and last until May 31. Under the order, prefectural governors asked residents to stay home and for some businesses to temporarily close, but public cooperation was voluntary. There were no penalties for failure to comply.

On May 13, Abe ended the state of emergency in 39 of the country's 47 prefectures. The decree remained in place for urban regions, including the capital, Tokyo, and the large port city of Osaka.

Abe lifted the state of emergency in Osaka as well as Kyoto and Hyogo on May 21, following a drop in the number of new cases reported each day. Chiba, Hokkaido, Kanagawa, Saitama and Tokyo all remained under restrictions until now.

The prime minister has credited the recent decline in new infections to the efforts of residents staying at home and practicing social distancing. He warned, however, that the state of emergency may have to be reimposed if infections increase.

More than 16,500 people in Japan have been diagnosed with COVID-19 and at least 820 have died, according to a count kept by Johns Hopkins University.

5:18 a.m.: US reports over 20,600 new coronavirus cases

More than 20,600 people in the United States were diagnosed with COVID-19 on Sunday, according to a count kept by Johns Hopkins University.

The country also reported over 600 new deaths from the disease on the same day.

The United States is, by far, the hardest-hit country in the coronavirus pandemic.

What to know about coronavirus:

3:45 a.m.: Houston receives hundreds of social distancing complaints over the weekend

Authorities in Houston said they have received hundreds of social distancing complaints over Memorial Day weekend amid the coronavirus pandemic.

Texas Gov. Greg Abbott's executive order allows bars to reopen at 25% of their normal capacity and restaurants at 50%. However, Houston Fire Chief Samuel Pena said via Twitter on Sunday afternoon that his department had addressed around 300 complaints of violation to the governor's rules since Friday, adding that "admittance beyond approved capacity will cause events to be stopped until condition is corrected."

Houston Mayor Sylvester Turner announced 115 new confirmed cases of COVID-19 with one additional death in the city on Sunday, bringing the total to 6,640 cases with 126 deaths.

Turner also said he saw photos and videos of people flouting the social distancing rules at crowded bars and packed pool parties over the holiday weekend. The mayor pleaded with businesses and customers to "be responsible," noting that their behavior puts first responders at risk, too.

"We don't want to be heavy handed," Turner said at a press conference Sunday. "If you work with us, nobody gets closed down."

ABC News' Clark Bentson, Marilyn Heck, Alina Lobzina and Ben Siegel contributed to this report.

Places of worship can hold services and funerals that limit attendance to 25% of a building's capacity, or up to 100 attendees, whichever is lower, if it gets approval from their local health departments. The places of worship must also follow other guidelines listed on the health department's website.

Store owners would also need approval from their local health departments before they can allow for in-person customers and they must also follow strict distancing rules. Personal services such as hair salons, nail salons and barbershops will remain closed, according to the new guidelines.

4:05 p.m.: UK to reopen stores starting June 1

British Prime Minister Boris Johnson said starting June 1, non-essential retail can begin to reopen.

The first group of stores that will be permitted will be outdoor markets and car show rooms. After June 15, all other non-essential retail, provided strict hygiene and social distancing measures are in place, will be permitted.

"These are careful but deliberate steps on the road to rebuilding our country -- and we can only take these thanks to what we have so far achieved together," Johnson said at a news conference.

12:50 p.m.: WHO suspends hydroxychloroquine study

The World Health Organization said it would pause its study on the anti-malaria drug hydroxychloroquine as part of its ongoing research into COVID-19 treatments.

The move comes after a study was published in medical journal The Lancet that found coronavirus patients who took the drug were more likely to die or develop irregular heart rhythms. WHO said its executive group of the Solidarity Trial is reviewing data on its research into the drug while the pause goes into effect.

"The other arms of the trial are continuing," Dr. Samba Sow, director general of the Center for Vaccine Development in Mali, said at a news conference. "This concern relates to the use of hydroxychloroquine and chloraquine in COVID-19."

12:05 p.m.: New York to provide death benefits to all public front-line workers

New York Gov. Andrew Cuomo announced that the state will provide death benefits to all workers on the front line who have died during the pandemic.

That includes police officers, EMTs, firefighters, hospital workers and transit employees.

"They showed up because I asked them to show up," Cuomo said at his daily briefing. "I have such respect and esteem for what they did … and I want to make sure we repay that."

Cuomo called on the federal government to approve a similar move for front-line workers across the nation.

He said the state's efforts to curb the number of cases continues to show promise. There were 96 COVID-19 deaths in the state within the last 24 hours, according to the governor's office.

That's a drop of 13 deaths recorded the previous day. The total number of hospitalizations was 4,348 on Sunday, which represented a 45 drop from Saturday, Cuomo's office said.

11:33 a.m.: Putin's spokesman reportedly discharged from hospital after coronavirus bout

Russian President Vladimir Putin's longtime press secretary, Dmitry Peskov, has been discharged from a hospital in Moscow after recovering from COVID-19, according to Russian media reports.

"Yes, that's true," Peskov told the independent Interfax news agency on Monday. He also confirmed that he must stay home for two weeks upon leaving the hospital.

On May 12, Peskov was quoted telling Russian state-run news agency RIA Novosti that he was hospitalized after testing positive for the novel coronavirus.

11:02 a.m.: Trump administration to buy 100 million swabs but leaves testing responsibility to states

The U.S. government plans to acquire 100 million swabs and distribute them to states to help expand the country's capacity to test for the novel coronavirus, according to a new report submitted to Congress and obtained by ABC News.

While the Trump administration was required to submit a national testing plan to Congress under a law passed last month, the 81-page report submitted Sunday doesn't appear to be a new strategy but rather a continuation of the White House's existing posture: that states should take the lead in expanding their own testing capacity. The Washington Post was first to report on the contents of the document.

"With support from the Federal government to ensure States are meeting goals, the State plans for testing will advance the safe reopening of America," the Trump administration wrote in the report's conclusion.

The U.S. government still projects the country will be able to produce 40 million to 50 million COVID-19 tests a month by September, and it recommends that every state should be able to test 2% of its population in May and June.

10:40 a.m.: Spain to end mandatory quarantine for international travelers in July

Spain will lift a two-week mandatory quarantine for all travelers arriving from abroad starting July 1, the government announced Monday.

"The worst is behind us," Spanish Foreign Minister Arancha Gonzalez Laya wrote on Twitter, adding that next month officials will "gradually" open to international tourists, lift the quarantine measures and "ensure the highest standards of health safety."

Spain is one of the worst-affected countries in the coronavirus pandemic, with more than 235,000 diagnosed cases of COVID-19 and at least 28,752 deaths, according to a count kept by Johns Hopkins University.

9:54 a.m.: Wuhan conducts over 6.5 million coronavirus tests in 10 days

The Chinese city of Wuhan, ground zero of the coronavirus pandemic, has conducted more than 6.5 million nucleic acid tests for the novel coronavirus over a 10-day period, according to state media reports.

Earlier this month, the Wuhan Municipal Health Commission launched a citywide campaign to test the entire population of 11 million residents for COVID-19 in an effort to search for asymptomatic carriers of the virus after a cluster of new cases emerged for the first time since the city had lifted its strict lockdown on April 8.

Although recommended, participation in the testing campaign is voluntary. Residents who were previously tested do not need to take part. It is not recommended to test children under the age of 6, according to the Wuhan Municipal Health Commission's website.

No new symptomatic cases of COVID-19 have been reported since the campaign began; however, dozens of people without symptoms have tested positive for the virus, according to daily reports published by the Wuhan Municipal Health Commission. The local health authority is asking anyone who hasn't been tested yet to come forward by the end of Tuesday.

Prior to the campaign, the city had completed over 3 million nucleic acid tests for COVID-19, according to the official state-run Xinhua News Agency.

9:06 a.m.: Austria completes 1st coronavirus lung transplant in Europe

A team of doctors in Austria's capital have successfully conducted a lung transplant on a coronavirus patient -- the first to be done in Europe.

The Medical University of Vienna announced the news in a statement Monday, saying its surgeons had carried out the procedure at Vienna General Hospital last week on a 45-year-old woman who had developed "severe respiratory failure" due to COVID-19. The team replaced the patient's lungs, "which had been damaged beyond repair," with those from a donor in what the university described as "an otherwise hopeless situation."

"We are very satisfied with the patient's condition, given the extremely difficult initial circumstances," Dr. Walter Klepetko, head of surgery at the clinic, said in a statement released by the university. "Only a few days after the procedure, the patient is well on the way to recovery."

The patient, from the southern state of Carinthia, had contracted the novel coronavirus about eight weeks ago. As her condition deteriorated, artificial ventilation was no longer possible and a circulation pump was the only thing keeping her alive, according to the university.

The woman was transported to Vienna General Hospital where she was admitted into the intensive care unit. A diagnostic test showed that particles of the novel coronavirus were still present in her system, but a negative viral culture subsequently confirmed she was no longer infectious. With no chance of the woman's lungs recovering but her other organs still functioning, the team of doctors decided to perform "an urgent and highly complex" transplant. The donor lungs were flown in from a neighboring country.

"The transplant itself took place under extremely difficult circumstances, since the patient did not have an adequate blood platelet count and, since antibodies were also present, these first had to be removed by means of immune aphaeresis to prevent her from rejecting the organ," Klepetko said. "Even transportation of the lungs and preparation for the operation took place under difficult conditions, especially because of the necessary COVID-19 logistics and the associated protective measures, which all had to be observed. In cases such of this, the key to success is smooth and effective collaboration between the various professional groups, such as anaesthesia, surgery, intensive care medicine, infectiology and many others."

8:38 a.m.: Montenegro declares itself virus-free

Montenegro has become the second country in Europe to declare itself free of the novel coronavirus.

Montenegro's Institute for Public Health announced via Twitter on Sunday that the tiny Balkan nation currently has no active cases of COVID-19. The news comes 68 days after Montenegro recorded its first case of the disease.

"As of yesterday, our laboratories have completed analysis of 140 respiratory samples, among which there were no new cases of infection," the institute tweeted. "At the same time, the last active case met the criteria for full recovery."

Montenegro, which is home to some 631,000 people, has reported 324 cases of COVID-19 with nine deaths, according to a count kept by Johns Hopkins University.

Earlier this month, Slovenia became the first European nation declare itself coronavirus-free.

7:10 a.m.: South Korea to require masks on transit, flights

People must wear face masks when using public transportation and taxis in South Korea starting Tuesday, as part of efforts to slow the spread of the novel coronavirus while public activities are increased.

South Korean health ministry official Yoon Taeho said masks will also be required on all domestic and international flights from Wednesday. Meanwhile, starting in June, owners of "high-risk" facilities such as bars, gyms and concert halls will be required to register customers at the door using smartphone QR codes to aid contact tracing when infections occur.

"Until treatments and vaccines are developed," we will never know when the COVID-19 crisis could end, and until then we will have to learn how to live with COVID-19," Yoon told reporters Monday.

South Korea once had the largest novel coronavirus outbreak outside China, where the virus first emerged, but appears to have brought it largely under control with an extensive "trace, test and treat" strategy. A total of 11,206 people in the country have been diagnosed with COVID-19, of which 10,226 have recovered and 267 have died, according to South Korea's Centers for Disease Control and Prevention.

Earlier this month, South Korea returned to a sense of normalcy as the nation eased its strict social-distancing measures that were put in place to curb the spread of the virus. The number of new cases reported in the country has generally stayed low, but health authorities remain wary of cluster infections and imported cases. More than 200 cases were recently linked to reopened bars, nightclubs and other entertainment venues in Seoul, the densely populated capital.

6:23 a.m.: Russia surpasses 350,000 coronavirus cases

Russia reported 8,946 new cases of COVID-19 on Monday, bringing the country's tally to 353,427.

Russia's coronavirus response headquarters also recorded 92 coronavirus-related deaths over the past 24 hours, placing the nationwide toll at 3,633, which is considerably lower than many other countries hit hard by the pandemic.

The daily rise in deaths has declined by a third after a record high of 153 reported on Sunday. Meanwhile, the latest daily caseload is down from a peak of 11,656 new infections reported on May 11, during which Russia registered over 10,000 new cases per day over a 12-day period. Since then, the daily number of new infections has hovered around 9,000 per day.

Over the weekend, Brazil surpassed Russia as the country with the second-highest number of diagnosed COVID-19 cases in the world, behind the United States, according to a count kept by Johns Hopkins University.

5:31 a.m.: Japan lifts state of emergency in Tokyo and other remaining areas

Japanese Prime Minister Shinzo Abe lifted the state of emergency in Tokyo and four other remaining areas on Monday, bringing an end to the restrictions nationwide.

A government-commissioned panel of experts approved the move in the prefectures of Tokyo, Saitama, Kanagawa, Hokkaido and Chiba, all of which had remained under the emergency declaration after it was lifted for most of Japan earlier this month.

The prime minister first declared a monthlong state of emergency for Tokyo and six other prefectures on April 7, as Japan reported a surge in COVID-19 cases. He later expanded the declaration to cover the entire country and last until May 31. Under the order, prefectural governors asked residents to stay home and for some businesses to temporarily close, but public cooperation was voluntary. There were no penalties for failure to comply.

On May 13, Abe ended the state of emergency in 39 of the country's 47 prefectures. The decree remained in place for urban regions, including the capital, Tokyo, and the large port city of Osaka.

Abe lifted the state of emergency in Osaka as well as Kyoto and Hyogo on May 21, following a drop in the number of new cases reported each day. Chiba, Hokkaido, Kanagawa, Saitama and Tokyo all remained under restrictions until now.

The prime minister has credited the recent decline in new infections to the efforts of residents staying at home and practicing social distancing. He warned, however, that the state of emergency may have to be reimposed if infections increase.

More than 16,500 people in Japan have been diagnosed with COVID-19 and at least 820 have died, according to a count kept by Johns Hopkins University.

5:18 a.m.: US reports over 20,600 new coronavirus cases

More than 20,600 people in the United States were diagnosed with COVID-19 on Sunday, according to a count kept by Johns Hopkins University.

The country also reported over 600 new deaths from the disease on the same day.

The United States is, by far, the hardest-hit country in the coronavirus pandemic.

What to know about coronavirus:

3:45 a.m.: Houston receives hundreds of social distancing complaints over the weekend

Authorities in Houston said they have received hundreds of social distancing complaints over Memorial Day weekend amid the coronavirus pandemic.

Texas Gov. Greg Abbott's executive order allows bars to reopen at 25% of their normal capacity and restaurants at 50%. However, Houston Fire Chief Samuel Pena said via Twitter on Sunday afternoon that his department had addressed around 300 complaints of violation to the governor's rules since Friday, adding that "admittance beyond approved capacity will cause events to be stopped until condition is corrected."

Houston Mayor Sylvester Turner announced 115 new confirmed cases of COVID-19 with one additional death in the city on Sunday, bringing the total to 6,640 cases with 126 deaths.

Turner also said he saw photos and videos of people flouting the social distancing rules at crowded bars and packed pool parties over the holiday weekend. The mayor pleaded with businesses and customers to "be responsible," noting that their behavior puts first responders at risk, too.

"We don't want to be heavy handed," Turner said at a press conference Sunday. "If you work with us, nobody gets closed down."

ABC News' Clark Bentson, Marilyn Heck, Alina Lobzina and Ben Siegel contributed to this report.

11:33 a.m.: Putin's spokesman reportedly discharged from hospital after coronavirus bout

Russian President Vladimir Putin's longtime press secretary, Dmitry Peskov, has been discharged from a hospital in Moscow after recovering from COVID-19, according to Russian media reports.

"Yes, that's true," Peskov told the independent Interfax news agency on Monday. He also confirmed that he must stay home for two weeks upon leaving the hospital.

On May 12, Peskov was quoted telling Russian state-run news agency RIA Novosti that he was hospitalized after testing positive for the novel coronavirus.

11:02 a.m.: Trump administration to buy 100 million swabs but leaves testing responsibility to states

The U.S. government plans to acquire 100 million swabs and distribute them to states to help expand the country's capacity to test for the novel coronavirus, according to a new report submitted to Congress and obtained by ABC News.

While the Trump administration was required to submit a national testing plan to Congress under a law passed last month, the 81-page report submitted Sunday doesn't appear to be a new strategy but rather a continuation of the White House's existing posture: that states should take the lead in expanding their own testing capacity. The Washington Post was first to report on the contents of the document.

"With support from the Federal government to ensure States are meeting goals, the State plans for testing will advance the safe reopening of America," the Trump administration wrote in the report's conclusion.

The U.S. government still projects the country will be able to produce 40 million to 50 million COVID-19 tests a month by September, and it recommends that every state should be able to test 2% of its population in May and June.

10:40 a.m.: Spain to end mandatory quarantine for international travelers in July

Spain will lift a two-week mandatory quarantine for all travelers arriving from abroad starting July 1, the government announced Monday.

"The worst is behind us," Spanish Foreign Minister Arancha Gonzalez Laya wrote on Twitter, adding that next month officials will "gradually" open to international tourists, lift the quarantine measures and "ensure the highest standards of health safety."

Spain is one of the worst-affected countries in the coronavirus pandemic, with more than 235,000 diagnosed cases of COVID-19 and at least 28,752 deaths, according to a count kept by Johns Hopkins University.

9:54 a.m.: Wuhan conducts over 6.5 million coronavirus tests in 10 days

The Chinese city of Wuhan, ground zero of the coronavirus pandemic, has conducted more than 6.5 million nucleic acid tests for the novel coronavirus over a 10-day period, according to state media reports.

Earlier this month, the Wuhan Municipal Health Commission launched a citywide campaign to test the entire population of 11 million residents for COVID-19 in an effort to search for asymptomatic carriers of the virus after a cluster of new cases emerged for the first time since the city had lifted its strict lockdown on April 8.

Although recommended, participation in the testing campaign is voluntary. Residents who were previously tested do not need to take part. It is not recommended to test children under the age of 6, according to the Wuhan Municipal Health Commission's website.

No new symptomatic cases of COVID-19 have been reported since the campaign began; however, dozens of people without symptoms have tested positive for the virus, according to daily reports published by the Wuhan Municipal Health Commission. The local health authority is asking anyone who hasn't been tested yet to come forward by the end of Tuesday.

Prior to the campaign, the city had completed over 3 million nucleic acid tests for COVID-19, according to the official state-run Xinhua News Agency.

9:06 a.m.: Austria completes 1st coronavirus lung transplant in Europe

A team of doctors in Austria's capital have successfully conducted a lung transplant on a coronavirus patient -- the first to be done in Europe.

The Medical University of Vienna announced the news in a statement Monday, saying its surgeons had carried out the procedure at Vienna General Hospital last week on a 45-year-old woman who had developed "severe respiratory failure" due to COVID-19. The team replaced the patient's lungs, "which had been damaged beyond repair," with those from a donor in what the university described as "an otherwise hopeless situation."

"We are very satisfied with the patient's condition, given the extremely difficult initial circumstances," Dr. Walter Klepetko, head of surgery at the clinic, said in a statement released by the university. "Only a few days after the procedure, the patient is well on the way to recovery."

The patient, from the southern state of Carinthia, had contracted the novel coronavirus about eight weeks ago. As her condition deteriorated, artificial ventilation was no longer possible and a circulation pump was the only thing keeping her alive, according to the university.

The woman was transported to Vienna General Hospital where she was admitted into the intensive care unit. A diagnostic test showed that particles of the novel coronavirus were still present in her system, but a negative viral culture subsequently confirmed she was no longer infectious. With no chance of the woman's lungs recovering but her other organs still functioning, the team of doctors decided to perform "an urgent and highly complex" transplant. The donor lungs were flown in from a neighboring country.

"The transplant itself took place under extremely difficult circumstances, since the patient did not have an adequate blood platelet count and, since antibodies were also present, these first had to be removed by means of immune aphaeresis to prevent her from rejecting the organ," Klepetko said. "Even transportation of the lungs and preparation for the operation took place under difficult conditions, especially because of the necessary COVID-19 logistics and the associated protective measures, which all had to be observed. In cases such of this, the key to success is smooth and effective collaboration between the various professional groups, such as anaesthesia, surgery, intensive care medicine, infectiology and many others."

8:38 a.m.: Montenegro declares itself virus-free

Montenegro has become the second country in Europe to declare itself free of the novel coronavirus.

Montenegro's Institute for Public Health announced via Twitter on Sunday that the tiny Balkan nation currently has no active cases of COVID-19. The news comes 68 days after Montenegro recorded its first case of the disease.

"As of yesterday, our laboratories have completed analysis of 140 respiratory samples, among which there were no new cases of infection," the institute tweeted. "At the same time, the last active case met the criteria for full recovery."

Montenegro, which is home to some 631,000 people, has reported 324 cases of COVID-19 with nine deaths, according to a count kept by Johns Hopkins University.

Earlier this month, Slovenia became the first European nation declare itself coronavirus-free.

7:10 a.m.: South Korea to require masks on transit, flights

People must wear face masks when using public transportation and taxis in South Korea starting Tuesday, as part of efforts to slow the spread of the novel coronavirus while public activities are increased.

South Korean health ministry official Yoon Taeho said masks will also be required on all domestic and international flights from Wednesday. Meanwhile, starting in June, owners of "high-risk" facilities such as bars, gyms and concert halls will be required to register customers at the door using smartphone QR codes to aid contact tracing when infections occur.

"Until treatments and vaccines are developed," we will never know when the COVID-19 crisis could end, and until then we will have to learn how to live with COVID-19," Yoon told reporters Monday.

South Korea once had the largest novel coronavirus outbreak outside China, where the virus first emerged, but appears to have brought it largely under control with an extensive "trace, test and treat" strategy. A total of 11,206 people in the country have been diagnosed with COVID-19, of which 10,226 have recovered and 267 have died, according to South Korea's Centers for Disease Control and Prevention.

Earlier this month, South Korea returned to a sense of normalcy as the nation eased its strict social-distancing measures that were put in place to curb the spread of the virus. The number of new cases reported in the country has generally stayed low, but health authorities remain wary of cluster infections and imported cases. More than 200 cases were recently linked to reopened bars, nightclubs and other entertainment venues in Seoul, the densely populated capital.

6:23 a.m.: Russia surpasses 350,000 coronavirus cases

Russia reported 8,946 new cases of COVID-19 on Monday, bringing the country's tally to 353,427.

Russia's coronavirus response headquarters also recorded 92 coronavirus-related deaths over the past 24 hours, placing the nationwide toll at 3,633, which is considerably lower than many other countries hit hard by the pandemic.

The daily rise in deaths has declined by a third after a record high of 153 reported on Sunday. Meanwhile, the latest daily caseload is down from a peak of 11,656 new infections reported on May 11, during which Russia registered over 10,000 new cases per day over a 12-day period. Since then, the daily number of new infections has hovered around 9,000 per day.

Over the weekend, Brazil surpassed Russia as the country with the second-highest number of diagnosed COVID-19 cases in the world, behind the United States, according to a count kept by Johns Hopkins University.

5:31 a.m.: Japan lifts state of emergency in Tokyo and other remaining areas

Japanese Prime Minister Shinzo Abe lifted the state of emergency in Tokyo and four other remaining areas on Monday, bringing an end to the restrictions nationwide.

A government-commissioned panel of experts approved the move in the prefectures of Tokyo, Saitama, Kanagawa, Hokkaido and Chiba, all of which had remained under the emergency declaration after it was lifted for most of Japan earlier this month.

The prime minister first declared a monthlong state of emergency for Tokyo and six other prefectures on April 7, as Japan reported a surge in COVID-19 cases. He later expanded the declaration to cover the entire country and last until May 31. Under the order, prefectural governors asked residents to stay home and for some businesses to temporarily close, but public cooperation was voluntary. There were no penalties for failure to comply.

On May 13, Abe ended the state of emergency in 39 of the country's 47 prefectures. The decree remained in place for urban regions, including the capital, Tokyo, and the large port city of Osaka.

Abe lifted the state of emergency in Osaka as well as Kyoto and Hyogo on May 21, following a drop in the number of new cases reported each day. Chiba, Hokkaido, Kanagawa, Saitama and Tokyo all remained under restrictions until now.

The prime minister has credited the recent decline in new infections to the efforts of residents staying at home and practicing social distancing. He warned, however, that the state of emergency may have to be reimposed if infections increase.

More than 16,500 people in Japan have been diagnosed with COVID-19 and at least 820 have died, according to a count kept by Johns Hopkins University.

5:18 a.m.: US reports over 20,600 new coronavirus cases

More than 20,600 people in the United States were diagnosed with COVID-19 on Sunday, according to a count kept by Johns Hopkins University.

The country also reported over 600 new deaths from the disease on the same day.

The United States is, by far, the hardest-hit country in the coronavirus pandemic.

What to know about coronavirus:

3:45 a.m.: Houston receives hundreds of social distancing complaints over the weekend

Authorities in Houston said they have received hundreds of social distancing complaints over Memorial Day weekend amid the coronavirus pandemic.

Texas Gov. Greg Abbott's executive order allows bars to reopen at 25% of their normal capacity and restaurants at 50%. However, Houston Fire Chief Samuel Pena said via Twitter on Sunday afternoon that his department had addressed around 300 complaints of violation to the governor's rules since Friday, adding that "admittance beyond approved capacity will cause events to be stopped until condition is corrected."

Houston Mayor Sylvester Turner announced 115 new confirmed cases of COVID-19 with one additional death in the city on Sunday, bringing the total to 6,640 cases with 126 deaths.

Turner also said he saw photos and videos of people flouting the social distancing rules at crowded bars and packed pool parties over the holiday weekend. The mayor pleaded with businesses and customers to "be responsible," noting that their behavior puts first responders at risk, too.

"We don't want to be heavy handed," Turner said at a press conference Sunday. "If you work with us, nobody gets closed down."

ABC News' Clark Bentson, Marilyn Heck, Alina Lobzina and Ben Siegel contributed to this report.

Earlier this month, the Wuhan Municipal Health Commission launched a citywide campaign to test the entire population of 11 million residents for COVID-19 in an effort to search for asymptomatic carriers of the virus after a cluster of new cases emerged for the first time since the city had lifted its strict lockdown on April 8.

Although recommended, participation in the testing campaign is voluntary. Residents who were previously tested do not need to take part. It is not recommended to test children under the age of 6, according to the Wuhan Municipal Health Commission's website.

No new symptomatic cases of COVID-19 have been reported since the campaign began; however, dozens of people without symptoms have tested positive for the virus, according to daily reports published by the Wuhan Municipal Health Commission. The local health authority is asking anyone who hasn't been tested yet to come forward by the end of Tuesday.

Prior to the campaign, the city had completed over 3 million nucleic acid tests for COVID-19, according to the official state-run Xinhua News Agency.

9:06 a.m.: Austria completes 1st coronavirus lung transplant in Europe

A team of doctors in Austria's capital have successfully conducted a lung transplant on a coronavirus patient -- the first to be done in Europe.

The Medical University of Vienna announced the news in a statement Monday, saying its surgeons had carried out the procedure at Vienna General Hospital last week on a 45-year-old woman who had developed "severe respiratory failure" due to COVID-19. The team replaced the patient's lungs, "which had been damaged beyond repair," with those from a donor in what the university described as "an otherwise hopeless situation."

"We are very satisfied with the patient's condition, given the extremely difficult initial circumstances," Dr. Walter Klepetko, head of surgery at the clinic, said in a statement released by the university. "Only a few days after the procedure, the patient is well on the way to recovery."

The patient, from the southern state of Carinthia, had contracted the novel coronavirus about eight weeks ago. As her condition deteriorated, artificial ventilation was no longer possible and a circulation pump was the only thing keeping her alive, according to the university.

The woman was transported to Vienna General Hospital where she was admitted into the intensive care unit. A diagnostic test showed that particles of the novel coronavirus were still present in her system, but a negative viral culture subsequently confirmed she was no longer infectious. With no chance of the woman's lungs recovering but her other organs still functioning, the team of doctors decided to perform "an urgent and highly complex" transplant. The donor lungs were flown in from a neighboring country.

"The transplant itself took place under extremely difficult circumstances, since the patient did not have an adequate blood platelet count and, since antibodies were also present, these first had to be removed by means of immune aphaeresis to prevent her from rejecting the organ," Klepetko said. "Even transportation of the lungs and preparation for the operation took place under difficult conditions, especially because of the necessary COVID-19 logistics and the associated protective measures, which all had to be observed. In cases such of this, the key to success is smooth and effective collaboration between the various professional groups, such as anaesthesia, surgery, intensive care medicine, infectiology and many others."

8:38 a.m.: Montenegro declares itself virus-free

Montenegro has become the second country in Europe to declare itself free of the novel coronavirus.

Montenegro's Institute for Public Health announced via Twitter on Sunday that the tiny Balkan nation currently has no active cases of COVID-19. The news comes 68 days after Montenegro recorded its first case of the disease.

"As of yesterday, our laboratories have completed analysis of 140 respiratory samples, among which there were no new cases of infection," the institute tweeted. "At the same time, the last active case met the criteria for full recovery."

Montenegro, which is home to some 631,000 people, has reported 324 cases of COVID-19 with nine deaths, according to a count kept by Johns Hopkins University.

Earlier this month, Slovenia became the first European nation declare itself coronavirus-free.

7:10 a.m.: South Korea to require masks on transit, flights

People must wear face masks when using public transportation and taxis in South Korea starting Tuesday, as part of efforts to slow the spread of the novel coronavirus while public activities are increased.

South Korean health ministry official Yoon Taeho said masks will also be required on all domestic and international flights from Wednesday. Meanwhile, starting in June, owners of "high-risk" facilities such as bars, gyms and concert halls will be required to register customers at the door using smartphone QR codes to aid contact tracing when infections occur.

"Until treatments and vaccines are developed," we will never know when the COVID-19 crisis could end, and until then we will have to learn how to live with COVID-19," Yoon told reporters Monday.

South Korea once had the largest novel coronavirus outbreak outside China, where the virus first emerged, but appears to have brought it largely under control with an extensive "trace, test and treat" strategy. A total of 11,206 people in the country have been diagnosed with COVID-19, of which 10,226 have recovered and 267 have died, according to South Korea's Centers for Disease Control and Prevention.

Earlier this month, South Korea returned to a sense of normalcy as the nation eased its strict social-distancing measures that were put in place to curb the spread of the virus. The number of new cases reported in the country has generally stayed low, but health authorities remain wary of cluster infections and imported cases. More than 200 cases were recently linked to reopened bars, nightclubs and other entertainment venues in Seoul, the densely populated capital.

6:23 a.m.: Russia surpasses 350,000 coronavirus cases

Russia reported 8,946 new cases of COVID-19 on Monday, bringing the country's tally to 353,427.

Russia's coronavirus response headquarters also recorded 92 coronavirus-related deaths over the past 24 hours, placing the nationwide toll at 3,633, which is considerably lower than many other countries hit hard by the pandemic.

The daily rise in deaths has declined by a third after a record high of 153 reported on Sunday. Meanwhile, the latest daily caseload is down from a peak of 11,656 new infections reported on May 11, during which Russia registered over 10,000 new cases per day over a 12-day period. Since then, the daily number of new infections has hovered around 9,000 per day.

Over the weekend, Brazil surpassed Russia as the country with the second-highest number of diagnosed COVID-19 cases in the world, behind the United States, according to a count kept by Johns Hopkins University.

5:31 a.m.: Japan lifts state of emergency in Tokyo and other remaining areas

Japanese Prime Minister Shinzo Abe lifted the state of emergency in Tokyo and four other remaining areas on Monday, bringing an end to the restrictions nationwide.

A government-commissioned panel of experts approved the move in the prefectures of Tokyo, Saitama, Kanagawa, Hokkaido and Chiba, all of which had remained under the emergency declaration after it was lifted for most of Japan earlier this month.

The prime minister first declared a monthlong state of emergency for Tokyo and six other prefectures on April 7, as Japan reported a surge in COVID-19 cases. He later expanded the declaration to cover the entire country and last until May 31. Under the order, prefectural governors asked residents to stay home and for some businesses to temporarily close, but public cooperation was voluntary. There were no penalties for failure to comply.

On May 13, Abe ended the state of emergency in 39 of the country's 47 prefectures. The decree remained in place for urban regions, including the capital, Tokyo, and the large port city of Osaka.

Abe lifted the state of emergency in Osaka as well as Kyoto and Hyogo on May 21, following a drop in the number of new cases reported each day. Chiba, Hokkaido, Kanagawa, Saitama and Tokyo all remained under restrictions until now.

The prime minister has credited the recent decline in new infections to the efforts of residents staying at home and practicing social distancing. He warned, however, that the state of emergency may have to be reimposed if infections increase.

More than 16,500 people in Japan have been diagnosed with COVID-19 and at least 820 have died, according to a count kept by Johns Hopkins University.

5:18 a.m.: US reports over 20,600 new coronavirus cases

More than 20,600 people in the United States were diagnosed with COVID-19 on Sunday, according to a count kept by Johns Hopkins University.

The country also reported over 600 new deaths from the disease on the same day.

The United States is, by far, the hardest-hit country in the coronavirus pandemic.

What to know about coronavirus:

3:45 a.m.: Houston receives hundreds of social distancing complaints over the weekend

Authorities in Houston said they have received hundreds of social distancing complaints over Memorial Day weekend amid the coronavirus pandemic.

Texas Gov. Greg Abbott's executive order allows bars to reopen at 25% of their normal capacity and restaurants at 50%. However, Houston Fire Chief Samuel Pena said via Twitter on Sunday afternoon that his department had addressed around 300 complaints of violation to the governor's rules since Friday, adding that "admittance beyond approved capacity will cause events to be stopped until condition is corrected."

Houston Mayor Sylvester Turner announced 115 new confirmed cases of COVID-19 with one additional death in the city on Sunday, bringing the total to 6,640 cases with 126 deaths.

Turner also said he saw photos and videos of people flouting the social distancing rules at crowded bars and packed pool parties over the holiday weekend. The mayor pleaded with businesses and customers to "be responsible," noting that their behavior puts first responders at risk, too.

"We don't want to be heavy handed," Turner said at a press conference Sunday. "If you work with us, nobody gets closed down."

ABC News' Clark Bentson, Marilyn Heck, Alina Lobzina and Ben Siegel contributed to this report.

8:38 a.m.: Montenegro declares itself virus-free

Montenegro has become the second country in Europe to declare itself free of the novel coronavirus.

Montenegro's Institute for Public Health announced via Twitter on Sunday that the tiny Balkan nation currently has no active cases of COVID-19. The news comes 68 days after Montenegro recorded its first case of the disease.

"As of yesterday, our laboratories have completed analysis of 140 respiratory samples, among which there were no new cases of infection," the institute tweeted. "At the same time, the last active case met the criteria for full recovery."

Montenegro, which is home to some 631,000 people, has reported 324 cases of COVID-19 with nine deaths, according to a count kept by Johns Hopkins University.

Earlier this month, Slovenia became the first European nation declare itself coronavirus-free.

7:10 a.m.: South Korea to require masks on transit, flights

People must wear face masks when using public transportation and taxis in South Korea starting Tuesday, as part of efforts to slow the spread of the novel coronavirus while public activities are increased.

South Korean health ministry official Yoon Taeho said masks will also be required on all domestic and international flights from Wednesday. Meanwhile, starting in June, owners of "high-risk" facilities such as bars, gyms and concert halls will be required to register customers at the door using smartphone QR codes to aid contact tracing when infections occur.

"Until treatments and vaccines are developed," we will never know when the COVID-19 crisis could end, and until then we will have to learn how to live with COVID-19," Yoon told reporters Monday.

South Korea once had the largest novel coronavirus outbreak outside China, where the virus first emerged, but appears to have brought it largely under control with an extensive "trace, test and treat" strategy. A total of 11,206 people in the country have been diagnosed with COVID-19, of which 10,226 have recovered and 267 have died, according to South Korea's Centers for Disease Control and Prevention.

Earlier this month, South Korea returned to a sense of normalcy as the nation eased its strict social-distancing measures that were put in place to curb the spread of the virus. The number of new cases reported in the country has generally stayed low, but health authorities remain wary of cluster infections and imported cases. More than 200 cases were recently linked to reopened bars, nightclubs and other entertainment venues in Seoul, the densely populated capital.

6:23 a.m.: Russia surpasses 350,000 coronavirus cases

Russia reported 8,946 new cases of COVID-19 on Monday, bringing the country's tally to 353,427.

Russia's coronavirus response headquarters also recorded 92 coronavirus-related deaths over the past 24 hours, placing the nationwide toll at 3,633, which is considerably lower than many other countries hit hard by the pandemic.

The daily rise in deaths has declined by a third after a record high of 153 reported on Sunday. Meanwhile, the latest daily caseload is down from a peak of 11,656 new infections reported on May 11, during which Russia registered over 10,000 new cases per day over a 12-day period. Since then, the daily number of new infections has hovered around 9,000 per day.

Over the weekend, Brazil surpassed Russia as the country with the second-highest number of diagnosed COVID-19 cases in the world, behind the United States, according to a count kept by Johns Hopkins University.

5:31 a.m.: Japan lifts state of emergency in Tokyo and other remaining areas

Japanese Prime Minister Shinzo Abe lifted the state of emergency in Tokyo and four other remaining areas on Monday, bringing an end to the restrictions nationwide.

A government-commissioned panel of experts approved the move in the prefectures of Tokyo, Saitama, Kanagawa, Hokkaido and Chiba, all of which had remained under the emergency declaration after it was lifted for most of Japan earlier this month.

The prime minister first declared a monthlong state of emergency for Tokyo and six other prefectures on April 7, as Japan reported a surge in COVID-19 cases. He later expanded the declaration to cover the entire country and last until May 31. Under the order, prefectural governors asked residents to stay home and for some businesses to temporarily close, but public cooperation was voluntary. There were no penalties for failure to comply.

On May 13, Abe ended the state of emergency in 39 of the country's 47 prefectures. The decree remained in place for urban regions, including the capital, Tokyo, and the large port city of Osaka.

Abe lifted the state of emergency in Osaka as well as Kyoto and Hyogo on May 21, following a drop in the number of new cases reported each day. Chiba, Hokkaido, Kanagawa, Saitama and Tokyo all remained under restrictions until now.

The prime minister has credited the recent decline in new infections to the efforts of residents staying at home and practicing social distancing. He warned, however, that the state of emergency may have to be reimposed if infections increase.

More than 16,500 people in Japan have been diagnosed with COVID-19 and at least 820 have died, according to a count kept by Johns Hopkins University.

5:18 a.m.: US reports over 20,600 new coronavirus cases

More than 20,600 people in the United States were diagnosed with COVID-19 on Sunday, according to a count kept by Johns Hopkins University.

The country also reported over 600 new deaths from the disease on the same day.

The United States is, by far, the hardest-hit country in the coronavirus pandemic.

What to know about coronavirus:

3:45 a.m.: Houston receives hundreds of social distancing complaints over the weekend

Authorities in Houston said they have received hundreds of social distancing complaints over Memorial Day weekend amid the coronavirus pandemic.

Texas Gov. Greg Abbott's executive order allows bars to reopen at 25% of their normal capacity and restaurants at 50%. However, Houston Fire Chief Samuel Pena said via Twitter on Sunday afternoon that his department had addressed around 300 complaints of violation to the governor's rules since Friday, adding that "admittance beyond approved capacity will cause events to be stopped until condition is corrected."

Houston Mayor Sylvester Turner announced 115 new confirmed cases of COVID-19 with one additional death in the city on Sunday, bringing the total to 6,640 cases with 126 deaths.

Turner also said he saw photos and videos of people flouting the social distancing rules at crowded bars and packed pool parties over the holiday weekend. The mayor pleaded with businesses and customers to "be responsible," noting that their behavior puts first responders at risk, too.

"We don't want to be heavy handed," Turner said at a press conference Sunday. "If you work with us, nobody gets closed down."

ABC News' Clark Bentson, Marilyn Heck, Alina Lobzina and Ben Siegel contributed to this report.

Over the weekend, Brazil surpassed Russia as the country with the second-highest number of diagnosed COVID-19 cases in the world, behind the United States, according to a count kept by Johns Hopkins University.

5:31 a.m.: Japan lifts state of emergency in Tokyo and other remaining areas

Japanese Prime Minister Shinzo Abe lifted the state of emergency in Tokyo and four other remaining areas on Monday, bringing an end to the restrictions nationwide.

A government-commissioned panel of experts approved the move in the prefectures of Tokyo, Saitama, Kanagawa, Hokkaido and Chiba, all of which had remained under the emergency declaration after it was lifted for most of Japan earlier this month.

The prime minister first declared a monthlong state of emergency for Tokyo and six other prefectures on April 7, as Japan reported a surge in COVID-19 cases. He later expanded the declaration to cover the entire country and last until May 31. Under the order, prefectural governors asked residents to stay home and for some businesses to temporarily close, but public cooperation was voluntary. There were no penalties for failure to comply.

On May 13, Abe ended the state of emergency in 39 of the country's 47 prefectures. The decree remained in place for urban regions, including the capital, Tokyo, and the large port city of Osaka.

Abe lifted the state of emergency in Osaka as well as Kyoto and Hyogo on May 21, following a drop in the number of new cases reported each day. Chiba, Hokkaido, Kanagawa, Saitama and Tokyo all remained under restrictions until now.

The prime minister has credited the recent decline in new infections to the efforts of residents staying at home and practicing social distancing. He warned, however, that the state of emergency may have to be reimposed if infections increase.

More than 16,500 people in Japan have been diagnosed with COVID-19 and at least 820 have died, according to a count kept by Johns Hopkins University.

5:18 a.m.: US reports over 20,600 new coronavirus cases

More than 20,600 people in the United States were diagnosed with COVID-19 on Sunday, according to a count kept by Johns Hopkins University.

The country also reported over 600 new deaths from the disease on the same day.

The United States is, by far, the hardest-hit country in the coronavirus pandemic.

What to know about coronavirus:

3:45 a.m.: Houston receives hundreds of social distancing complaints over the weekend

Authorities in Houston said they have received hundreds of social distancing complaints over Memorial Day weekend amid the coronavirus pandemic.

Texas Gov. Greg Abbott's executive order allows bars to reopen at 25% of their normal capacity and restaurants at 50%. However, Houston Fire Chief Samuel Pena said via Twitter on Sunday afternoon that his department had addressed around 300 complaints of violation to the governor's rules since Friday, adding that "admittance beyond approved capacity will cause events to be stopped until condition is corrected."

Houston Mayor Sylvester Turner announced 115 new confirmed cases of COVID-19 with one additional death in the city on Sunday, bringing the total to 6,640 cases with 126 deaths.

Turner also said he saw photos and videos of people flouting the social distancing rules at crowded bars and packed pool parties over the holiday weekend. The mayor pleaded with businesses and customers to "be responsible," noting that their behavior puts first responders at risk, too.

"We don't want to be heavy handed," Turner said at a press conference Sunday. "If you work with us, nobody gets closed down."

ABC News' Clark Bentson, Marilyn Heck, Alina Lobzina and Ben Siegel contributed to this report.

A government-commissioned panel of experts approved the move in the prefectures of Tokyo, Saitama, Kanagawa, Hokkaido and Chiba, all of which had remained under the emergency declaration after it was lifted for most of Japan earlier this month.

The prime minister first declared a monthlong state of emergency for Tokyo and six other prefectures on April 7, as Japan reported a surge in COVID-19 cases. He later expanded the declaration to cover the entire country and last until May 31. Under the order, prefectural governors asked residents to stay home and for some businesses to temporarily close, but public cooperation was voluntary. There were no penalties for failure to comply.

On May 13, Abe ended the state of emergency in 39 of the country's 47 prefectures. The decree remained in place for urban regions, including the capital, Tokyo, and the large port city of Osaka.

Abe lifted the state of emergency in Osaka as well as Kyoto and Hyogo on May 21, following a drop in the number of new cases reported each day. Chiba, Hokkaido, Kanagawa, Saitama and Tokyo all remained under restrictions until now.

The prime minister has credited the recent decline in new infections to the efforts of residents staying at home and practicing social distancing. He warned, however, that the state of emergency may have to be reimposed if infections increase.

More than 16,500 people in Japan have been diagnosed with COVID-19 and at least 820 have died, according to a count kept by Johns Hopkins University.

5:18 a.m.: US reports over 20,600 new coronavirus cases

More than 20,600 people in the United States were diagnosed with COVID-19 on Sunday, according to a count kept by Johns Hopkins University.

The country also reported over 600 new deaths from the disease on the same day.

The United States is, by far, the hardest-hit country in the coronavirus pandemic.

What to know about coronavirus:

3:45 a.m.: Houston receives hundreds of social distancing complaints over the weekend

Authorities in Houston said they have received hundreds of social distancing complaints over Memorial Day weekend amid the coronavirus pandemic.

Texas Gov. Greg Abbott's executive order allows bars to reopen at 25% of their normal capacity and restaurants at 50%. However, Houston Fire Chief Samuel Pena said via Twitter on Sunday afternoon that his department had addressed around 300 complaints of violation to the governor's rules since Friday, adding that "admittance beyond approved capacity will cause events to be stopped until condition is corrected."

Houston Mayor Sylvester Turner announced 115 new confirmed cases of COVID-19 with one additional death in the city on Sunday, bringing the total to 6,640 cases with 126 deaths.

Turner also said he saw photos and videos of people flouting the social distancing rules at crowded bars and packed pool parties over the holiday weekend. The mayor pleaded with businesses and customers to "be responsible," noting that their behavior puts first responders at risk, too.

"We don't want to be heavy handed," Turner said at a press conference Sunday. "If you work with us, nobody gets closed down."

ABC News' Clark Bentson, Marilyn Heck, Alina Lobzina and Ben Siegel contributed to this report.

The prime minister has credited the recent decline in new infections to the efforts of residents staying at home and practicing social distancing. He warned, however, that the state of emergency may have to be reimposed if infections increase.

More than 16,500 people in Japan have been diagnosed with COVID-19 and at least 820 have died, according to a count kept by Johns Hopkins University.

5:18 a.m.: US reports over 20,600 new coronavirus cases

More than 20,600 people in the United States were diagnosed with COVID-19 on Sunday, according to a count kept by Johns Hopkins University.

The country also reported over 600 new deaths from the disease on the same day.

The United States is, by far, the hardest-hit country in the coronavirus pandemic.

What to know about coronavirus:

3:45 a.m.: Houston receives hundreds of social distancing complaints over the weekend

Authorities in Houston said they have received hundreds of social distancing complaints over Memorial Day weekend amid the coronavirus pandemic.

Texas Gov. Greg Abbott's executive order allows bars to reopen at 25% of their normal capacity and restaurants at 50%. However, Houston Fire Chief Samuel Pena said via Twitter on Sunday afternoon that his department had addressed around 300 complaints of violation to the governor's rules since Friday, adding that "admittance beyond approved capacity will cause events to be stopped until condition is corrected."

Houston Mayor Sylvester Turner announced 115 new confirmed cases of COVID-19 with one additional death in the city on Sunday, bringing the total to 6,640 cases with 126 deaths.

Turner also said he saw photos and videos of people flouting the social distancing rules at crowded bars and packed pool parties over the holiday weekend. The mayor pleaded with businesses and customers to "be responsible," noting that their behavior puts first responders at risk, too.

"We don't want to be heavy handed," Turner said at a press conference Sunday. "If you work with us, nobody gets closed down."

ABC News' Clark Bentson, Marilyn Heck, Alina Lobzina and Ben Siegel contributed to this report.

3:45 a.m.: Houston receives hundreds of social distancing complaints over the weekend

Authorities in Houston said they have received hundreds of social distancing complaints over Memorial Day weekend amid the coronavirus pandemic.

Texas Gov. Greg Abbott's executive order allows bars to reopen at 25% of their normal capacity and restaurants at 50%. However, Houston Fire Chief Samuel Pena said via Twitter on Sunday afternoon that his department had addressed around 300 complaints of violation to the governor's rules since Friday, adding that "admittance beyond approved capacity will cause events to be stopped until condition is corrected."

Houston Mayor Sylvester Turner announced 115 new confirmed cases of COVID-19 with one additional death in the city on Sunday, bringing the total to 6,640 cases with 126 deaths.

Turner also said he saw photos and videos of people flouting the social distancing rules at crowded bars and packed pool parties over the holiday weekend. The mayor pleaded with businesses and customers to "be responsible," noting that their behavior puts first responders at risk, too.

"We don't want to be heavy handed," Turner said at a press conference Sunday. "If you work with us, nobody gets closed down."

ABC News' Clark Bentson, Marilyn Heck, Alina Lobzina and Ben Siegel contributed to this report.

Turner also said he saw photos and videos of people flouting the social distancing rules at crowded bars and packed pool parties over the holiday weekend. The mayor pleaded with businesses and customers to "be responsible," noting that their behavior puts first responders at risk, too.

"We don't want to be heavy handed," Turner said at a press conference Sunday. "If you work with us, nobody gets closed down."

ABC News' Clark Bentson, Marilyn Heck, Alina Lobzina and Ben Siegel contributed to this report.