New York (AFP) - Over 4,500 flights were cancelled around the world by Saturday and thousands more were delayed as the highly infectious Omicron variant disrupted holiday travel, according to tracking website Flightaware.

One route, however, was thankfully not affected: Santa was on track delivering presents to children across the globe, air defense officials assured US President Joe Biden.

According to Flightaware.com, at least 2,000 flights were cancelled worldwide on Christmas Day, including roughly 700 originating from or headed to US airports, with more than 1,500 delays as of 0720 GMT.

On Friday, there were around 2,400 cancellations and almost 11,000 delays. The site also reported over 600 cancellations for Sunday.

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

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

"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 on 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, adding that it was working to rebook passengers.

Similarly, Delta cancelled at least 260 flights Saturday and around 170 on Friday, saying it has "exhausted all options and resources -- including rerouting and substitutions of aircraft and crews to cover scheduled flying."

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

Eleven Alaska Airlines flights were axed, after employees said they were potentially exposed to Covid-19 and had to self-isolate in quarantine.

The cancellations added to the pandemic frustration for many Americans 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 around 480 flights or over 20 percent of it's flight plan, and Air China grounding 15 percent of its total scheduled departures at around 0720 GMT Saturday.

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 January 2, a 34 percent increase since last year.

But most of those plans were made before the outbreak of the Omicron variant, which has become the dominant strain in the United States, overwhelming some hospitals and healthcare workers.

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 over 60 years.

Major General Eric Kenny, commander of the Canadian NORAD region, told AFP that Santa had distributed over two billion gifts and was above Pakistan around 1800 GMT.

"Santa is doing very well so far," Kenny said. "He's been at it for many hours already and will go throughout the evening as well."

Biden and First Lady Jill Biden also spoke to NORAD officials via video conference Friday to ensure Father Christmas' trip was going smoothly.

Biden then spoke, also via video call, to several American families, asking children what presents they wanted and telling them that they had to be in bed before midnight, otherwise Santa would not come.

But in a sign of deep political divisions plaguing American society, the event then took a sour turn, when a father, to whom Biden had just wished happy holidays, launched an insult at him.

According to journalists present at the event, the father said "Merry Christmas" followed by "Let's go Brandon!" The phrase has been used by supporters of Donald Trump as a euphemism for a derogatory remark against the current president.