More than 720 commercial flights going into or out of the US were cancelled on Sunday, according to a tally on flight-tracking website FlightAware.com.

This meant thousands of people were forced to make alternative Christmas travel arrangements as airlines cancelled flights so pilots and cabin crew could quarantine during one of the busiest times of the year.

Cases have risen in many parts of the US over the festive period, which prompted New York state's health department to warn on Friday there had been four-fold increase in COVID hospital admissions for children under 18 since the week beginning December 5.

Delta Air Lines Inc predicted more than 300 of its flights to be cancelled by the end of Sunday.

A Delta spokesperson said: "Winter weather in portions of the US and the Omicron variant continued to impact Delta's holiday weekend flight schedule."

They added the airline was working to "reroute and substitute aircraft and crews to get customers where they need to be as quickly and safely as possible".

However, if this is not possible, the spokesperson said it was coordinating with impacted customers on the next available flight.

United Airlines also called off 98 flights today, with a spokesperson adding that a small portion were able to rebook alternative flights.

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Spokesperson Maddie King said: "Importantly, 25% of customers whose travel was interrupted were able to rebook on flights that allowed them to get to their final destination earlier than they otherwise would have."

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

The official said: "But any cancellations can be a pain and delay reunions with family and friends, so the Transportation Department and the Federal Aviation Administration are monitoring this closely."