‘This is deadly serious’: Biden calls for states to reinstate mask mandates amid uptick in COVID-19 cases

President Joe Biden warned state officials who lifted mask mandates to reinstate them as well as maintain any preventative health measures to stave off a potential fourth COVID-19 surge, Monday.

"If we let our guard down now, we can see the virus getting worse, not better. People are letting up on precautions, which is a very bad thing. Please, this is not politics, reinstate the mandate if you let it down," Biden said."Now is not the time to let down," Biden continued.

"Now’s not the time to celebrate. It is time to do what we do best as a country: our duty, our jobs, take care of one another.""Fight to the finish," he added. "Don’t let up now."

Biden COVID-19 response

FILE - Vice President Kamala Harris looks on as U.S. President Joe Biden delivers remarks on the COVID-19 response and the state of vaccinations in the South Court Auditorium at the White House complex on March 29, 2021 in Washington, DC.

RELATED: ‘Right now, I’m scared’: CDC director feels ‘impending doom’ amid COVID-19 uptick in US

The warning came even as Biden laid out hopeful new steps to expand coronavirus vaccinations, with all adults to become eligible over the next 5 weeks. Biden announced plans to expand the number of retail pharmacies that are administering vaccines, and investments to help Americans get to vaccination sites. But the optimism was tempered by stark warnings about the potential for another wave of cases.

"This is deadly serious," Biden said.

Biden announced that the U.S. is expecting a delivery of 33 million doses of COVID-19 vaccine this week — including 11 million of the single-dose Johnson & Johnson shot.

On Thursday, the U.S. set a new single-day record for shots in arms: more than 3.2 million.

RELATED: Real-world study finds Moderna, Pfizer COVID-19 vaccines 90% effective after 2 doses, CDC says

Hours earlier, during a virtual White House health briefing, Dr. Rochelle Walensky, director of the U.S. Centers for Disease Control and Prevention, grew emotional as she reflected on her experience treating COVID-19 patients who are alone at the end of their lives.

"We have so much to look forward to, so much promise and potential of where we are and so much reason for hope," she said. "But right now, I’m scared."

"I’m going to lose the script, and I’m going to reflect on the recurring feeling I have of impending doom," she continued.

"We are doing things prematurely," Dr. Anthony Fauci, the nation’s top infectious disease expert, said, referring to moves to ease up on restrictions.

RELATED: Fauci warns of possible coronavirus surge in U.S.

Despite the over 51 million Americans who have been inoculated, the seven-day average for positive COVID-19 cases rose nearly 10.6% over the past week, according to the CDC. The seven-day average for hospitalizations rose by 4.2% over the past week and the seven-day COVID-19 death rate average has increased 2.6%.

"We are not powerless, we can change this trajectory of the pandemic," Walensky said, pointing to an uptick in travel and loosening virus restrictions for the increase in cases.

"People want to be done with this. I, too, want to be done with this," she said. "Just please hold on a little while longer."

"We’ve seen surges after every single holiday," she reiterated. "Please limit travel to essential travel for the time being."

The Associated Press and Chris Williams contributed to this report.