Editor’s Note: This story has been updated to include a statement from Delta Air Lines.

(NEXSTAR) – Two major airlines have canceled dozens of flights scheduled for Christmas Eve and Christmas day, largely due to the omicron variant.

Delta Air Lines and United Airlines together canceled more than 600 flights on Friday and Saturday. As of Saturday morning, Delta canceled 284 flights, according to FlightAware. (Other factors, such as weather, are also causing cancellations.) United called off 238 flights on Saturday, about 12% of its schedule. Planned cancellations continued into Sunday.

United Airlines shared a statement with Nexstar saying, “The nationwide spike in Omicron cases this week has had a direct impact on our flight crews and the people who run our operation. As a result, we’ve unfortunately had to cancel some flights and are notifying impacted customers in advance of them coming to the airport. We’re sorry for the disruption and are working hard to rebook as many people as possible and get them on their way for the holidays.”

In a statement shared with Nexstar, the Delta said “Delta teams have exhausted all options and resources — including rerouting and substitutions of aircraft and crews to cover scheduled flying — before canceling around 90 flights for Friday. We apologize to our customers for the delay in their holiday travel plans. Delta people are working hard to get them to where they need to be as quickly and as safely as possible on the next available flight.”

A spokesperson says the cancelations are due to a number of issues, including potential inclement weather and the impact of the omicron variant.

Omicron recently became the dominant COVID variant in the U.S. As of Thursday, the U.S. has surpassed its summer peak of COVID infections with a seven-day average of more than 168,000 cases, according to data from The New York Times.

Not all airlines said COVID was disrupting their travel schedules. American Airlines said it had “nothing to report,” while Southwest Airlines said “things are running smoothly.” JetBlue did not respond to a request for comment.

Last week, the president of the nation’s largest flight attendant union and executives at United Airlines, American Airlines, Southwest Airlines, and Delta Air Lines testified during a Senate Commerce Committee hearing on airline oversight. During the hearing, Southwest Airlines CEO Gary Kelly told the Senate panel that the air in passenger jets’ cabins is so clean that face masks “don’t add much” additional protection against the spread of COVID-19 on planes, The Hill reports. Kelly tested positive for COVID-19 days after the hearing.

Watch travel expert Peter Greenberg discuss what travelers need to be prepared for.