United Airlines planes sit on the runway at Newark Liberty International Airport on November 30, in Newark, New Jersey. (Spencer Platt/Getty Images)

“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 Christmas Eve flights according to flight tracking site FlightAware. 

United says 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."