Top US pandemic advisor Anthony Fauci on Sunday warned of a bleak winter ahead as the Covid-19 Omicron variant spurs a new wave of infections globally, sparking restrictions and concerns over hospital capacity.

"One thing that's very clear... is its (Omicron's) extraordinary capability of spreading," Fauci told NBC News. "It is just... raging through to the world."

Since it was first reported in South Africa in November, Omicron has been identified in dozens of countries, prompting many to reimpose travel restrictions and other measures.

Despite indications it is not more severe than the Delta variant -- currently still the dominant strain -- the heavily mutated Omicron has been shown in early data to have a worrying resistance to vaccines and higher transmissibility.

Fauci also cautioned against too much optimism over Omicron's severity, noting that in South Africa, while the hospitalization-to-case ratio is lower than with Delta, this could be due to underlying immunity from widespread previous infections.

"No matter how you look at it," he underscored, "when you have so many, many infections, even if it is less severe, that overcomes this slight to moderate diminution in severity, because our hospitals, if things look like they're looking now in the next week or two are going to be very stressed," particularly in areas of the country with low levels of vaccination.

The top scientist urged unvaccinated Americans to get a shot and the vaccinated to get boosters, which have been shown to re-up protection.

The administration of President Joe Biden, who is due to address the nation on pandemic developments on Tuesday, has been campaigning hard for vaccination.

While a little over 70 per cent of the US population has had at least one shot, according to the Centers for Disease Control and Prevention, another 50 million eligible people remain unprotected, Fauci said.

"It's never too late to get vaccinated, and if you're vaccinated, go get boosted," Fauci told CNN, adding that continuing to wear masks and get regular testing -- another area the Biden administration is investing in -- are also key to avoiding infection.

"With Omicron... it is going to be a tough few weeks to months as we get deeper into the winter."

Omicron now accounts for around three percent of cases in the United States, a figure that is expected to rise rapidly as has been seen in other countries.

On Saturday, New York state announced a record number of daily cases for the second day in a row with almost 22,000 positive results.

The United States is the nation hit hardest by the pandemic, crossing 800,000 known Covid-19 deaths on Tuesday, according to a Johns Hopkins University tracker.