Travellers wait at the check-in counter for Southwest Airlines in the terminal of Denver International Airport on Friday. AP

According to Flightaware.com, more than 2,500 flights were scrubbed around the globe on Christmas Day, including more than 870 originating from or headed to US airports, with some 4,200 delays as of 1430 GMT.

On Friday, there were around 2,400 cancellations and 11,000 delays, while Sunday cancellations have already topped 800.

Pilots, flight attendants and other employees have been calling in sick or having to quarantine after exposure to Covid, forcing Lufthansa, Delta, United Airlines, JetBlue and many other short-staffed carriers to cancel flights during one of the year's peak travel periods.

"Help @united flight cancelled again. I want to get home for Christmas," one exasperated traveler from the US state of Vermont tweeted to the airline early on Saturday.

Flightaware data showed United cancelled around 200 flights each day Friday and Saturday, or 10 percent of those that were scheduled.

A scramble to reroute pilots and planes and reassign employees was underway, but Omicron's surge has upended business.

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

"As a result, we've unfortunately had to cancel some flights and are notifying impacted customers in advance of them coming to the airport," the airline said.

Similarly, Delta scrapped at least 280 flights Saturday and was already cancelling 64 on Sunday, saying it has "exhausted all options and resources — including rerouting and substitutions of aircraft and crews to cover scheduled flying."

"We apologise to our customers for the delay in their holiday travel plans," the company said.

The cancellations added to the pandemic frustration for many people eager to reunite with their families over the holidays, after last year's Christmas was severely curtailed.

Chinese airlines accounted for the highest number of cancellations, with China Eastern scrapping 1,000 flights, over 20 percent of its flight plan, on Friday and Saturday and Air China also grounding about 20 percent of its scheduled departures over the period.

Huge travel bump

According to estimates from the American Automobile Association, more than 109 million Americans were scheduled to travel by plane, train or automobile between December 23 and Jan. 2, a 34 per cent increase over last year.

But most of those plans were made before the outbreak of Omicron, which has become the dominant strain in the United States, overwhelming some hospitals and healthcare workers.
The state of New York announced Friday that it recorded 44,431 new daily positive Covid tests, a new record.

Luckily, Omicron did not affect Santa's travel plans, according to the North American Aerospace Defense Command (NORAD), which has tracked St. Nick's Christmas journey for six decades.
US President Joe Biden and First Lady Jill Biden spoke to NORAD officials Friday to ensure Father Christmas's trip was going smoothly.

Biden then spoke with several American families via video call to discuss the holiday. But the lighthearted moment turned sour, exposing the country's political divisions, when a father to whom Biden had just wished happy holidays launched an insult at him.

The father replied "Merry Christmas," but followed up with "Let's go Brandon," a phrase used by Donald Trump supporters as a euphemism for a derogatory remark against the current president.