President Joe Biden said his administration has "to do better" to meet COVID-19 testing demands.

During the White House COVID-19 Response Team’s call with the National Governors Association Monday, the president directly addressed the shortages of kits being reported across the nation.

He said the steps the government had taken so far to make more COVID tests available is "not enough."

"If I had known, we would have gone harder, quicker if we could have," Biden said on the call.

He went on, "Seeing how tough it was for some folks to get a test this weekend shows we have more work to do and we’re doing it. We have to do more, we have to do better, and we will."

Dr. Anthony Fauci said a COVID vaccine requirement for domestic air travel should be "seriously" considered.

"If you're talking about requiring vaccination to get on a plane domestically, that is just another one of the requirements that I think is reasonable to consider," he said Monday on MSNBC's "Morning Joe."

"When you make vaccination a requirement, that's another incentive to get more people vaccinated. If you want to do that with domestic flights, I think that's something that seriously should be considered," he said.

This is not the first time that Fauci has argued for vaccine mandates domestic flights.

On Sunday, Fauci told ABC's Jon Karl that "anything that could get people more vaccinated would be welcome."

-ABC News' Arielle Mitropoulos

Mayor Bill de Blasio said New York City has administered more than 180,000 COVID vaccine booster shots since a $100 incentive was announced on Tuesday.

This brings the total number of residents who have received boosters to more than 1.9 million.

"We did a booster incentive and it has been heard and felt by the people of NYC," de Blasio told MSNBC on Monday.

-ABC News' Aaron Katersky

At least four U.S. cruise ships are reporting COVID-19 outbreaks aboard, leading to them being denied entry to -- or turned away from -- foreign ports.

Holland America's Line Koningsdam was denied entry into Puerto Vallarta on Thursday after 21 crew members tested positive.

Additionally, the Carnival Freedom ship was turned away from the Caribbean Islands of Bonaire and Aruba on Wednesday and Thursday, respectively, after "a small number" of passengers aboard tested positive for the virus.

Officials in Aruba and Curacao also turned away Royal Caribbean's Odyssey of the Seas on Wednesday after 55 fully vaccinated crew members and passengers tested positive.

On the same day, passengers aboard the Seven Seas Mariner were not allowed to disembark in Cartagena, Colombia, after six crew members and one passenger tested positive for COVID-19.

Dr. Anthony Fauci strongly recommended against large New Year's Eve parties at Wednesday's White House briefing.

"Should you change or cancel your plans? If your plans are to go to a 40-50 person New Year's Eve party with all the bells and whistles and everybody hugging and kissing and wishing each other a happy new year, I would strongly recommend that this year, we do not do that," Fauci said.

Fauci said "all indications point to a lesser severity of omicron versus delta." But he warned, "we should not become complacent since our hospital system could still be stressed in certain areas."

White House COVID-19 coordinator Jeff Zients said, "We have directly helped more than 30 states and territories by deploying over 2,100 federal personnel and thousands of ambulances, ventilators and other critical supplies."

Supplies include gloves, masks, respirators and face shields, he said.

He added, "More than 13,000 National Guard members have been activated in 48 states to support the COVID response, from vaccinations, to testing, to clinical care."

At the briefing, CDC Director Rochelle Walensky defended the agency's shortened isolation guidelines.

"We do know the vast majority of viral transmission happens in those first five days, somewhere in the 85 to 90% range. So if a person can isolate for the first five days they absolutely should," she said. "We also don't know that antigen tests give a good indication of transmissibility at this stage of infection. On the other hand, we know that after five days people are much less likely to transmit the virus and that masking further reduces that risk. And this is why people need to mask for five days after the five days of isolation."

After five days, asymptomatic people with COVID-19 can leave isolation, but must wear masks around others, according to the new guidelines.

-ABC News' Cheyenne Haslett