Over a thousand U.S. flights were canceled on Monday, adding to the at least 6,000 flights that have been delayed globally since Christmas Eve.

According to flight tracker FlightAware, airlines canceled more than 1,300 U.S. flights and delayed nearly 6,800 on Monday by 9:00 p.m. EST. There were over 14,000 global flight delays.

Seattle-Tacoma International Airport had the highest volume of canceled or delayed flights, canceling 125 flights and delaying another 215.

SkyWest, a Utah-based airline, had the highest amount of disruptions for U.S. airlines, with 416 cancelations and 780 delays.

The cancelations also impacted major airlines such as Alaska, American, Delta, and United.

Travelers queue up at the Southwest Airlines curbside check-in area at Denver International Airport Sunday, Dec. 26, 2021, in Denver. Airlines canceled hundreds of flights Sunday, citing staffing problems tied to COVID-19 to extend the nation’s travel problems beyond Christmas. (AP Photo/David Zalubowski)

The airlines chalked up the delays and cancelations to call-outs related to the omicron strain of the coronavirus or poor weather conditions.

“The nationwide spike in omicron cases has had a direct impact on our flight crews and the people who run our operation,” United said. “As a result, we’ve unfortunately had to cancel some flights and are notifying impacted customers in advance of them coming to the airport”

John Laughter, Delta Airlines Vice President and Chief of Operations, said the situation was “a perfect storm that includes relentless weather systems coupled with the omicron variant surge.”

Alaska Airlines cited winter weather in the Pacific Northwest as the reason for its 167 cancelations, which made up 24 percent of its Monday operations.

“We’re working as quickly as possible to get all our affected guests rebooked on other flights, while operating safely,” Alaska Airlines said. “Reservations is experiencing extremely long hold times of up to 11 hours.”

Monday’s disruptions continue the streak of cancelations and delays that began on Christmas Eve. According to ABC7, “A combined 1,700 flights were canceled on Christmas Eve and Christmas Day.” Additionally, 1,517 U.S. flights were canceled on Sunday.

Unfortunately, the cancelations occurred during one of the busiest travel periods for Americans. The Transportation Security Administration (TSA) screened 2.19 million travelers on December 23 and over 1.53 million on Christmas Day.