Omicron grounds hundreds of US flights

A total of 913 Christmas Day flights, domestic and international, were cancelled, up from 690 on Christmas Eve, according to a running tally on flight-tracking website FlightAware.com. 

More than 1100 flights were delayed.

The Christmas holidays are typically a peak time for air travel, but the rapid spread of the highly transmissible Omicron variant of COVID-19 has led to a sharp increase in infections, forcing airlines to cancel flights as pilots and crew need to quarantine.

United Airlines cancelled 230 flights, a company official said.

"The nationwide spike in Omicron cases this week has had a direct impact on our flight crews and the people who run our operation," spokeswoman Maddie King said. 

"We are working hard to rebook as many people as possible and get them on their way for the holidays."

FlightAware data showed Delta Air Lines scrubbed 301 flights as of 6am AEDT, while American Airlines said they had to call off 90 mainland flights. 

Globally, more than 2590 flights were called off on Saturday, with 5438 others delayed.

Among the most impacted US airports were Atlanta's Hartford-Jackson International, New Jersey's Newark Liberty International, Los Angeles International and New York's JFK International.

The Omicron variant was first detected in November and now accounts for nearly three-quarters of US cases, and as many as 90 per cent in some areas, such as the eastern seaboard.

The average number of new US coronavirus cases has risen 45 per cent to 179,000 a day over the past week. 

While recent research suggests Omicron produces milder illness and a lower rate of hospitalisation than previous variants of COVID-19, health officials have maintained a cautious note about the outlook.

Ahead of the Christmas holiday, Americans scrambled for COVID-19 tests and many went ahead with their travel plans.

US officials have said people who are fully vaccinated should feel comfortable proceeding with holiday travel.