The United States is experiencing "almost a vertical increase" in Covid cases as the Omicron variant sweeps the country, but the peak may be only weeks away, top US pandemic advisor Anthony Fauci said Sunday.

"We are definitely in the middle of a very severe surge and uptick in cases," Fauci said on ABC's "This Week," calling the soaring infection rate "really unprecedented."

With the Omicron variant of the virus sweeping around the world, more than 440,000 new cases were reported in the US on Friday, almost exactly 200,000 more than during a peak last February.

But Fauci said the experience of South Africa -- where the strain was first detected in late November and peaked quickly, then subsided almost as quickly -- offered some hope.

Evidence is mounting, he added, that Omicron is milder than previous variants. The US rates of deaths and hospitalizations have been far lower in recent weeks than during previous Covid surges.

The United States, like other countries, has been struggling to find a balance that will protect public health without gravely damaging the economy or slamming key services like policing and air travel.

With children set to return to school Monday following the year-end break, both Fauci and the US education secretary said they thought in-person instruction can be conducted safely if proper precautions are taken.

Fauci again pleaded with parents to be sure their children are vaccinated, wear masks and get tested if need be.

Top US pandemic advisor Anthony Fauci, seen here at a briefing on December 27, 2021, says a near 'vertical' increase in Covid cases may be short-lived
Top US pandemic advisor Anthony Fauci, seen here at a briefing on December 27, 2021, says a near 'vertical' increase in Covid cases may be short-lived GETTY IMAGES NORTH AMERICA via AFP / Anna Moneymaker

"I think all those things put together, it's safe enough to get those kids back to school, balanced against the deleterious effects of keeping them out," Fauci said.

US Education Secretary Miguel Cardona, meanwhile, said the return to the classroom would be challenging, but necessary.

"I do think there will be bumps in the road, especially tomorrow," he told "Fox News Sunday," with large numbers of teachers and staff calling in sick.

"So we are going to roll up our sleeves, all hands on deck, let's keep our children in the classroom. That should be our default thinking."

And Eric Adams, who was sworn in as New York mayor just minutes into the new year, said there was little choice but for children to return -- safely -- to school.

"We've lost almost two years of education," he said on ABC. "We can't do it again... The safest place for children is inside a school."