Washington, Dec 27 (IANS): Delta Air Lines, United Airlines, JetBlue Airways and American Airlines have cancelled more than 750 flights combined on Christmas Day, and cancellations were set to continue, upending plans during one of the busiest travel periods of the year, according to US media outlets.

"All four airlines said Omicron cases among staff were driving cancellations," Xinhua news agency quoted a USA Today report as saying on Sunday.

In a statement, Delta Air Lines said that "a combination of issues, including but not limited to inclement weather in some areas of the country and the impact of the Omicron variant, are driving cancellations and potential delays".

"Holiday travel is generally a stressful enterprise, but a rapid surge in cases of Covid-19 caused by the Omicron variant have caused hundreds of flight cancellations, adding another layer of difficulty to the proceedings," an NBC News report said on Sunday, noting that several major airlines are dealing with a shortage of workers.

CNN said that "major US airlines cancelled hundreds of more flights on Sunday, the third day in a row of mass cancellations and delays over Christmas weekend, as staff and crew call out sick amid the Omicron surge", adding that almost 700 flights were cancelled and another 1,300 were delayed on Sunday.

Delta Air Lines said it expected to cancel more than 300 flights, on top of 368 scratched on Saturday.

United Airlines cancelled nearly 100 mainline flights on Sunday due to staffing concerns, while roughly 25 per cent of its customers have been able to re-book to arrive earlier than their original plan.

JetBlue spokesperson Derek Dombrowski was quoted as saying that the airline has seen an "increasing number" of sick calls due to the fast-spreading Omicron variant, despite entering the holiday season with the highest staffing levels since the start of the pandemic.

American Airlines spokesperson Derek Walls said the company was "working hard" to re-book customers quickly.

United spokesperson Maddie King said the airline was also working to re-book as many people as possible "and get them on their way for the holidays".

Globally, airlines cancelled more than 6,000 flights on Christmas Eve, Christmas and the day after Christmas, according to FlightAware, a flight tracker website.

That included about 1,700 flights within, into or out of the US.

In an effort to head off staffing shortages and flight cancellations, US carriers have asked the Centers for Disease Control and Prevention to reduce recommended isolation periods for fully vaccinated people recovering from Cvid-19, The Wall Street Journal reported on Sunday.