More than 2,500 flights have been canceled Monday as COVID cases surge across the globe.Of the more than 2,500 canceled flights, 1,000 were within, into or out of the United States, according to FlightAware. More than 9,000 flights are delayed.Globally, airlines canceled more than 6,000 flights on Christmas Eve, Christmas and the day after Christmas. In the United States, more than 1,200 flights were canceled and more than 5,000 were delayed on Sunday alone as staff and crew call out sick.The cancellations come at the busiest time of year for air travel. The U.S. Transportation Security Administration said it screened millions of people each day over the holiday weekend, peaking at 2.19 million travelers on Thursday, Dec. 23. On Wednesday, more people passed through TSA checkpoints than on the same day in 2019.Alaska Airlines, which canceled 133 flights of 19% of its operations Monday, attributed its cancellations and delays on winter weather in the Pacific Northwest. The airline said it canceled almost 250 mainline flights scheduled to arrive or depart from Seattle Sunday. As of Monday afternoon, Seattle-Tacoma International Airport has the most delays and cancellations in the world, according to FlightAware."We're working as quickly as possible to get all our affected guests rebooked on other flights, while operating safely," Alaska Airlines said in a statement. "Reservations is experiencing extremely long hold times of up to 11 hours." Saturday air travel was a bit slower because of the flight cancellations: More than 1.53 million people passed through security checkpoints Saturday.United Airlines said last week it had to cancel hundreds of flights because it lacked enough crew members to safely fly all of its scheduled routes."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.Delta said it was working to get all stranded travelers home as quickly as possible."We apologize to our customers for the delay in their holiday travel plans," Delta said in a statement. "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."CNN's Pete Muntean, Arnaud Siad and Ramishah Maruf contributed to this report