Commercial airlines had cancelled 1001 flights within, into or out of the United States by mid-afternoon, according to a tally on flight-tracking website FlightAware.com.

It was the third straight day of travelling pain and further cancellations were likely as COVID-19 infections have soared, driven by the highly transmissible Omicron variant.

A total of 997 flights were scrapped on Christmas Day and nearly 700 on Christmas Eve. Thousands more were delayed on all three days.

The Christmas holidays, typically a peak time for travel, coincided with a rapid spread of the Omicron variant.

Infections have risen sharply in many parts of the country, with New York state's health department warning on Friday that it recorded a "startling" four-fold increase in COVID-19 hospital admissions for children under 18 since the week beginning December 5.

With the surge in infections, airlines have been forced to cancel flights with pilots and cabin crew needing to quarantine. Poor weather in some areas also contributed to the problems.

"Winter weather in portions of the US and the Omicron variant continued to impact Delta's holiday weekend flight schedule," a spokesperson for the airline said in an emailed statement, adding that it was working to "reroute and substitute aircraft and crews to get customers where they need to be as quickly and safely as possible."

A White House official, who asked not to be named, said that despite the mess at some airports, "we're in a better place than last Christmas" and noted that "only a small percentage of flights are affected."

The US airports most heavily impacted by the cancellations were in Seattle, Atlanta, Los Angeles, Dallas-Fort Worth and JFK International in New York.

Omicron 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 per day over the past week, according to a Reuters tally.

While recent research suggests Omicron produces milder illness and a lower rate of hospitalisations than previous iterations, health officials have maintained a cautious outlook and say much remains to be learned about the variant.

New York's acting state health commissioner Dr Mary Bassett said the "striking increase " in hospitalisations of children with COVID that the state has recorded in the last three weeks showed urgent action was needed.

All children older than five should be fully vaccinated, she said, while those under five should be shielded by ensuring those around them have protection through vaccination, boosters, mask-wearing, avoiding crowds and testing.

"The risks of COVID-19 for children are real," Bassett said in a statement. The state's health department said the increases were concentrated in New York City and surrounding areas where Omicron is spreading rapidly.

It said that in the most recent week, no five to 11-year-old admitted to the hospital due to COVID was fully vaccinated.

Australian Associated Press