Top US pandemic advisor Anthony Fauci on Sunday acknowledged a Covid "testing problem" as the Omicron strain overwhelms the nation, and he vowed to make more tests available to Americans next month.

"One of the problems is that that's not going to be totally available to everyone until we get to January, and there are still some issues now of people having trouble getting tested," Fauci told ABC talk show "This Week."

"But we're addressing the testing problem," he added, saying it should be corrected "very soon."

Covid cases have skyrocketed across the United States in recent weeks as the highly infectious Omicron variant became the dominant strain, overwhelming hospitals and Covid testing sites as Americans rushed to get tested for holiday travel.

Authorities have recorded an average of over 175,000 new daily cases over the recent period, according to data from the Centers for Disease Control and Prevention.

Omicron has forced the cancellation of hundreds of flights in the United States as crews called in sick or had to quarantine after exposure to the virus and airlines scrambled to rebook frustrated passengers who had hoped to spend Christmas with their families.

Fauci said the administration was ramping up in order to cope.

"The president's multi-part component of the response is to make sure that we have adequate backup for hospitals with military personnel, doctors, nurses and other health care providers, making sure that there's enough PPE (personal protective equipment) and that if needed, there's enough ventilators in the national strategic stockpile," he said.

The White House adviser also stressed that Omicron was "extraordinarily contagious."

At the same time, Fauci noted that recent studies in South Africa and Britain indicate that Omicron was less likely to lead to hospitalization than the previous strains of the virus and that the duration of hospital stays and oxygen needs for patients were lower.

But Fauci also warned that Omicron's apparent lower severity is likely to be neutralized by how fast it is spreading.

"The issue that we don't want to get complacent about... is that when you have such a high volume of new infections, it might override a real diminution in severity," Fauci said.

On Tuesday, the White House announced its new Covid response plan, which includes additional aid to hospitals and ramping up vaccination and testing, but no new restrictions ahead of the end-of-year holidays.