Photo taken on Dec. 4, 2021 shows a passenger wearing a face mask taking the escalator at Ronald Reagan Washington National Airport in Arlington, Virginia, the United States. (Photo by Aaron Schwartz/Xinhua)

Delta Air Lines, United Airlines, JetBlue Airways and American Airlines canceled more than 750 flights combined on Christmas Day, and cancellations were set to drag on through Sunday, upending plans during one of the busiest travel periods of the year, major U.S. media reported.

"All four airlines said Omicron cases among staff were driving cancellations," said USA Today on Sunday. "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," said Delta in a statement.

"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," reported NBC, noting that several major airlines are dealing with a shortage of workers.

"Major U.S. airlines canceled 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," reported CNN, adding that almost 700 U.S. flights were canceled and another 1,300 were delayed on Sunday.

MORE FLIGHT CANCELLATIONS

Delta Air Lines said it expected to cancel more than 300 flights on Sunday, on top of 368 scratched on Saturday. United Airlines canceled nearly 100 mainline flights on Sunday due to staffing concerns, while roughly 25 percent 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 Airlines 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 canceled 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 United States.

MORE PANDEMIC CASES

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

The flight cancellations, roughly 740 on Sunday after nearly 1,000 on Christmas Day, came as U.S. officials focused on ensuring there were enough staff and resources to make sure "we don't get an overrun on hospitals," Anthony Fauci, U.S. President Joe Biden's chief medical adviser, was quoted as saying.

"The president's multipart component of the response is to make sure that we have adequate backup for hospitals with military personnel, doctors, nurses and other healthcare providers, making sure that there's enough (personal protective equipment) and that if needed, there's enough ventilators in the national strategic stockpile," Fauci said.

"As of Sunday, the seven-day average of COVID-19 cases eclipsed the peak set during the Delta variant's earlier march through the country," said the report, noting that the average reached 184,302 as of Dec. 25, according to Johns Hopkins University data.