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.

The Centers for Disease Control and Prevention (CDC) warned Americans to avoid cruises regardless of COVID vaccination status.

On Thursday, the CDC raised the health notice from a level 3 to a level 4, which is the highest level.

"Since the identification of the omicron variant, there has been an increase in the number of COVID-19 cases among cruise passengers and crew reported to CDC," the agency wrote. "Additionally, there has been an increase in the number of cruise ships meeting the COVID-19 case threshold for CDC investigation."

More than 80 cruise ships are currently being investigated or observed by the CDC after reports of COVID outbreaks on board.

In response, the Cruise Lines International Association -- the largest cruise trade association -- called the CDC's new warning "particularly perplexing" and said cases on ships "consistently make up a very slim minority of the total population onboard."

ABC News' Alexandra Faul, Mina Kaji and Sam Sweeney