Originally Published: 27 DEC 21 07:37 ET

Updated: 27 DEC 21 11:32 ET

By Chris Liakos and Sonnet Swire, CNN Business

(CNN) -- More than 2,000 flights have been canceled Monday as Covid cases surge across the globe.

Of the more than 2,400 canceled flights, nearly 900 were within, into or out of the United States, according to FlightAware. More than 6,500 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 US Transportation Security Administration said it screened millions of people each day over the holiday weekend, peaking at 2.19 million travelers on Thursday, December 23. On Wednesday, more people passed through TSA checkpoints than on the same day in 2019.

Alaska Airlines, which canceled 105 flights of 15% of its operations Monday, attributed its cancellations and delays on winter weather in the Pacific Northwest.

"Snow and wintry conditions are creating a bit of a bah-humbug for our operations to and from Seattle, including flight cancellations and delays," Alaska Airlines said on its blog Sunday.

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."

International flights

European airlines are also experiencing a small number of cancellations amid record-breaking numbers of Covid-19 cases in several European nations.

A British Airways spokesperson told CNN on Monday that the airline had canceled "a number of flights due to operational constraints," ​and are instead using larger aircrafts, where possible, to get customers to their destinations. According to tracking website FlightAware, 46 flights from British Airways were canceled on Monday.

Virgin Atlantic told CNN flying continues "as scheduled," noting the exception of one of their London Heathrow -- New York JFK rotations which saw cancellations on December 21. A Virgin Atlantic spokesperson told CNN: "We continue to take pre-emptive measures to uphold operational and staffing resilience, always putting the health and safety of our customers and people first."

German airline Lufthansa said it will cancel 10% of its winter flight schedule as the pandemic continues to hit the aviation industry.

In an interview with Frankfurter Allgemeine Sonntagszeitung last week, Lufthansa CEO Carsten Spohr said that due to "a sharp drop in bookings" the airline will have to cancel 33,000 flights from mid-January to February 2022 or 10% of the group's winter flight schedule.

Spohr's comments were confirmed to CNN by the Lufthansa press office.