Maine and New Hampshire are now averaging more new cases than at any other point in the pandemic, while daily cases in Massachusetts, Vermont, Connecticut, Pennsylvania and Rhode Island are nearing peak levels, according to federal data.

Five of those states -- Connecticut, Maine, Massachusetts, Rhode Island and Vermont -- have the highest full vaccination rates in the country. 

In Florida, which has been largely spared from the latest COVID-19 wave, daily cases have increased by 92% over the last two weeks, according to federal data.

-ABC News' Arielle Mitropoulos

COVID-19 cases in New York City have tripled in the last month, officials warned Thursday.

Mayor Bill de Blasio announced a six-point plan to fight the surge, including increasing testing capacity, doubling down on business inspections and distributing 1 million KN95 masks and 500,000 rapid home tests.

"We need to stop this variant," the mayor said. "This variant moves fast. We need to move a lot faster."

-ABC News' Aaron Katersky

Dr. Anthony Fauci, the nation's leading expert on infectious diseases, warned Thursday that omicron will become the dominant variant of the novel coronavirus in the United States "very soon."

"It has an extraordinary ability to transmit efficiently and spread," Fauci, the chief medical advisor to U.S. President Joe Biden, told ABC News' George Stephanopoulos in an interview on "Good Morning America."

"It has what we call a doubling time of about three days and if you do the math on that, if you have just a couple of percentage of the isolates being omicron, very soon it's going to be the dominant variant," he explained. "We've seen that in South Africa, we're seeing it in the U.K. and I'm absolutely certain that's what we're going to be seeing here relatively soon."

Fauci, who is also the director of the National Institute of Allergy and Infectious Diseases, urged Americans to "absolutely" get vaccinated against COVID-19, if they haven't already, and to also receive a booster shot when they become eligible.

"At this point, we don't believe you need an omicron-specific boost," he added. "We just need to get the boost with what you got originally for the primary vaccination."

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Fauci talks importance of boosters as omicron variant grows in US

Dr. Anthony Fauci shares the latest information about the fast-growing omicron variant as COVID-19 cases continue to rise heading into the holidays.

France announced Thursday that it will ban non-essential travel to and from the United Kingdom due to the country's surge in cases of the omicron variant.

Starting Saturday, France will require people to have "a compelling reason" to travel between the two countries. Travel for tourism or work will not be allowed. French citizens, however, can return to France, according to a statement from the French prime minister's office.

All travelers from the U.K. will be required to show proof of a negative COVID-19 test taken less than 24 hours before departure. Upon arrival in France, they must self-isolate for a week, but that period can be ended after 48 hours if they test negative for COVID-19 again.

The new rules apply to people regardless of their vaccination status.

"Faced with the extremely rapid spread of the Omicron variant in the United Kingdom, the Government has chosen to reinstate the need for an essential reason for travel from and to the United Kingdom, and to strengthen the requirement for tests on departure and arrival," the French prime minister's office said in the statement Thursday. "The Government is also calling on travelers who had planned to visit the United Kingdom to postpone their trip."