(CNN) Airlines around the world have canceled more than 2,000 Christmas Eve flights. About 400 of those are in the U.S.
Major carriers say they have staffing shortages because of the COVID-19 Omicron variant.
According to the flight tracking site FlightAware, United has canceled 170 flights as of Friday morning. The airline has apologized and says it is working to rebook customers before they get to the airport.
Delta is also apologizing after canceling 130 flights.
These operational snags come as airlines see an uptick in passengers.
The Transportation Security Administration screened more than two million people on Thursday, the highest figure since holiday travel started a week ago.