A flurry of flight cancellations continued on Christmas Day, with U.S. airlines scrapping nearly 1,000 flights as the rapid spread of the Omicron variant of the coronavirus caused crew members to call out sick.

The cancellations affected more than 10 percent of the flights scheduled by Delta Air Lines, United Airlines and JetBlue. American Airlines had canceled less than 5 percent of flights, according to FlightAware, which provides aviation data.

By midday on Saturday the number of cancellations had already exceeded the total cancellations on Christmas Eve, a setback for travelers and airlines hoping this holiday season would be a return to relative normal. Globally, more than 2,500 flights scheduled for Christmas Day were canceled.

“A number of Covid-related sick calls led us to make the difficult decision to precancel some flights scheduled for today,” said Derek Walls, a spokesman for American Airlines, which had about 90 cancellations on Saturday. “We proactively notified affected customers yesterday, and are working hard to rebook them quickly.”

And the problems are expected to continue. Delta, which said weather in some parts of the country was also causing problems, expected to cancel more than 300 flights scheduled for Sunday. And FlightAware said other U.S. airlines had already canceled 150 flights for Dec. 26.

The cancellations come during one of the busiest travel periods of the year, and Americans are flying nearly as much as they were prepandemic, with about 2 million people streaming through screening checkpoints each day in the week leading up to Christmas Eve, according to the Transportation Security Administration. At some points the number of travelers this year even exceeded the number of passengers who flew in 2019, according to the T.S.A.’s data.

The travel period also came during a fresh surge of Covid cases. The U.S. is averaging almost 200,000 new cases each day, according to The New York Times’s coronavirus tracker, more than the average case load during this summer’s peak.

An airline trade group asked the Centers for Disease Control and Prevention on Thursday to shorten the recommended isolation period for employees who test positive for Covid-19 from 10 days to “no more than five days,” with a negative test to return. “The Omicron surge may exacerbate personnel shortages and create significant disruptions to our work force and operations,” the group, Airlines for America, wrote in a letter to the C.D.C’s director.

But the Association of Flight Attendants has pushed back on that request, telling the C.D.C. on Thursday that “we support your agency’s current recommendation to isolate for 10 days” and that decisions to reduce isolation times “should be made by public health professionals, not airlines.”