(CNN) — At least two major airlines in the United States have canceled flights just before Christmas Eve.

On Thursday, United Airlines said it had to “cancel some flights” because of the Omicron variant of the coronavirus.

“The nationwide spike in Omicron cases this week has had a direct impact on our flight crews and the people who run our operation,” said a United memo obtained by CNN.

United has canceled more than 100 Friday flights, according to flight tracking site FlightAware.

United said it is “notifying impacted customers in advance of them coming to the airport,” according to a company statement. “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.”

Later Thursday night, Delta Air Lines also canceled flights. The airline canceled 93 Christmas Eve flights, according to FlightAware. That’s 4% of its total schedule.

Delta said the cancellations are due to multiple issues including the Omicron variant.