Flight cancellations in the United States have dragged the “Christmas flightmare” experienced by travelers over the Christmas period into the new year.

According to FlightAware, over 3,300 flights globally have been canceled on Sunday so far, while nearly 4,400 flights around the world were cancelled on Saturday. Of these, 2,500 flights were in the U.S. on Saturday, and 1,910 were in the U.S. on Sunday.

Chicago O’Hare International Airport has been hardest hit, with 256 flights cancelled, amounting to 25% of all flights. Denver International Airport and Newark International Airport has seen numerous cancellations, as has Atlanta’s Hartsfield-Jackson International Airport, New York’s JFK and Boston’s Logan International Airport.

As was the case over the Christmas holiday, airlines have been struggling with staffing numbers due to the spread of COVID-19, as well as bad weather in the United States. Since December 24, more than 12,000 flights have been cancelled in the U.S. alone.