Palestinian authorities have imposed a 48-hour lockdown on the besieged Gaza Strip after officials said a family of four tested positive for COVID-19 at a refugee camp there.

The Palestinian health ministry said another case was identified Monday at a quarantine center in Gaza, bringing the total count to five.

The cause of the cluster is unclear and authorities are working to trace it. It’s the first known instance of the novel coronavirus spreading through Gaza’s local community.

The full lockdown went into effect Monday night across the entire 140-square-mile-long coastal enclave, which shares borders with Egypt and Israel.

The Gaza Strip, a densely populated area that forms part of the so-called Occupied Palestinian Territories, is inhabited by some 2 million Palestinians, most of whom are registered refugees living in overcrowded camps. Both Egypt and Israel have imposed restrictions on movement in and out of Gaza, citing concerns over the militant group Hamas, which rules the territory and has been designated a terrorist organization by Israel and the United States.

However, a small number of Palestinians still travel to Jerusalem for routine cancer treatments, and Egypt partially lifted its blockade recently to allow Palestinians back in.

ABC News’ Nasser Atta and Guy Davies contributed to this report.

The University of Southern California is reporting an “alarming increase in the number of COVID-19 cases” among students living in off-campus housing.

In a letter sent to students Monday, the university’s student health center said it had identified 43 cases in the past seven days. All cases are related to students in off-campus living environments, and more than 100 students are now in a 14-day quarantine due to exposures.

“This increase comes despite the continued state and county health guidance that significantly restricts in-person instruction and on-campus activities for universities located in counties that are on the state’s COVID-19 monitoring list, including Los Angeles County,” the letter states. “We continue to strongly discourage students from returning to the campus area until further notice.”

The private university, located in the heart of Los Angeles, kicked off its fall semester online last week, but many students remain in apartments and off-campus residences near the campus.

The numbers of new COVID-19 cases and new deaths in the United States have both decreased by substantial amounts in week-over-week comparisons, according to an internal memo from the Federal Emergency Management Agency obtained by ABC News Monday night.

Just five U.S. states and territories are currently in an upward trajectory of new cases, while five states are at a plateau and 46 states are going down, the memo said.

There were 300,366 new cases confirmed during the period of Aug. 17-23, a 16.2% decrease from the previous seven-day period. There were also 6,873 new deaths recorded Aug. 17-23, an 8.5% decrease from the previous week, according to the memo.

Meanwhile, the national positivity rate for COVID-19 tests ticked downward slightly from 6.4% to 5.5%, the memo said.

Oregon appears to be coming down from a peak with a 7% decrease in infections, a 19.5% decrease in hospitalizations and a 20% reduction in deaths for the week ending on Aug. 16, compared to the previous week, according to the memo.

Idaho has seen an 8.5% decrease in the seven-day case rate since Aug. 18, but a 12.2% increase in the seven-day death rate, the memo said.

In Montana, the Native American community makes up 7% of the state’s population but accounts for 18% of cases statewide and 34% of deaths, according to the memo.

ABC News’ Josh Margolin contributed to this report.

There were 38,045 new cases of COVID-19 identified in the United States on Monday, according to a count kept by Johns Hopkins University.

Monday's tally is well below the country's record set on July 16, when 77,255 new cases were identified in a 24-hour reporting period.

An additional 450 coronavirus-related deaths were also recorded Monday. The latest daily death toll is under the record 2,666 new fatalities that were reported on April 17.

It's the second consecutive day that the country has reported less than 40,000 new cases and fewer than 500 new deaths.

A total of 5,740,909 people in the United States have been diagnosed with COVID-19 since the pandemic began, and at least 177,279 of them have died, according to Johns Hopkins. The cases include people from all 50 U.S. states, Washington, D.C. and other U.S. territories as well as repatriated citizens.

By May 20, all U.S. states had begun lifting stay-at-home orders and other restrictions put in place to curb the spread of the novel coronavirus. The day-to-day increase in the country's cases then hovered around 20,000 for a couple of weeks before shooting back up and crossing 70,000 for the first time in mid-July.

Face masks will be obligatory for children aged 6 and older while at schools in Spain's central capital.

Madrid's regional premier, Isabel Diaz Ayuso, announced the new measure at a press conference Tuesday, while presenting a strategy for the safe return to schools amid the coronavirus pandemic. She said class sizes will also be reduced and thousands more teachers will be hired on a temporary basis.

Diaz Ayuso noted that, if Madrid's epidemiological situation improves, face masks will only be mandatory for students aged 11 and up. The region is planning for the staggered reopening of schools in September.

With over 405,000 diagnosed cases of COVID-19 so far, Spain has the highest case count of any country in Europe. The day-to-day increase of new cases has been steadily climbing since Spain emerged from a more than three-month national lockdown in late June.

Last week, Spain's head of health emergencies, Dr. Fernando Simon, warned that "things are not going well" regarding a recent spike of infections.

"Each day, we are seeing more and more transmission," Simon said.