The president also revealed more testing sites will be set up - as he warned the US is facing a "critical moment" in the pandemic.

And 1,000 military medical personnel will be deployed as part of efforts to help overwhelmed hospitals.

Around 40 million adults in the US have yet to get their vaccine - as Mr Biden said it was their "patriotic duty".

In a White House address to the nation, the president said: "While COVID-19 has been a tough adversary we have shown we are tougher. Tougher because we have the power of science and vaccines that prevent illness and save lives."

But he warned people who have yet to get the jab: "You have an obligation to yourselves, to your family and quite frankly, I know I will be criticised for this, but to your country.

"I honest to God believe it's your patriotic duty."

And he also pleaded with the "tens of millions" who have been double-vaccinated but not yet boosted to come forward.

"I got my booster shot as soon as it was available and just the other day, President Trump announced he had got his booster shot - maybe one of the few things he and I agree on."

Getting vaccinated is "the only responsible thing to do", he said.

President Biden spoke out as a member of his own staff tested positive for the coronavirus, the White House announced on Monday.

The employee spent around 30 minutes on Air Force One while the president was travelling to Philadelphia from Orangeburg, South Carolina on Friday, before developing symptoms on Sunday night.

The staff member is fully vaccinated and boosted and tested negative before boarding the jet, White House press secretary, Jen Psaki, said.

President Biden, 79, is tested regularly and has produced two negative results since Sunday. He will be tested again on Wednesday.

In his speech on Tuesday evening, the commander in chief said "dozens" of ambulances would be sent to New York and Maine which have seen a surge in cases.

Maine has battling a huge increase in daily cases in the past fortnight - up from 624 on 5 December to 915 cases on 19 December.

More than 70% of people are said to be vaccinated - but lower rates in rural areas have sparked further outbreaks, officials said.

The state has recorded more than 137,000 positive cases since the start of the pandemic, together with 1,400 deaths including 22 on Tuesday, according to the Maine Center for Disease Control and Prevention.

Meanwhile in New York City, the Omicron variant has also caused mayhem on Broadway with musicals including Aladdin and Hamilton announcing temporary closures in the face of "extreme uncertainty ahead".

Around 42,000 tested positive between Wednesday and Saturday - compared with fewer than 35,800 in all of November.

In Boston, up to 500 members of the National Guard will be deployed to help hospitals battling a surge of COVID-19 patients.

They will be tasked with providing support in key areas including security, transporting patients and delivering food to their rooms.

And in Florida, Omicron has overtaken the Delta variant in Miami-Dade County as the dominant strain, as Mayor Daniella Levine Cava said it was "absolutely astonishing" how contagious the variant has proved to be.

"It's on us to protect ourselves and still the most important thing we can do is vaccinate," she added.