As pre-pandemic level crowds hit airports for the the holidays, four major U.S. airlines have canceled more than 900 Christmas Day flights due to the fast-spreading omicron variant of COVID-19.

As of Saturday afternoon, Delta Airlines, United Airlines, JetBlue Airways and American Airlines had canceled a combined 950 Saturday flights.

Delta Air Lines canceled 308 flights on Christmas Day and has already canceled 69 flights scheduled for Sunday.

Saturday's cancellations come after United, Delta and JetBlue canceled 454 Christmas Eve flights.

Delta Air Lines canceled 173 flights for Christmas Eve. The airline says the "flight cancellations are due to a combination of issues, including but not limited to, potential inclement weather in some areas and the impact of the omicron variant."

"Delta teams have exhausted all options and resources -- including rerouting and substitutions of aircraft and crews to cover scheduled flying -- before canceling around 90 flights for Friday," Delta said in a Friday statement to ABC News. "We apologize to our customers for the delay in their holiday travel plans. 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."

PHOTO: A United Airlines airplane is pushed back from its gate at Newark Liberty International Airport as the sun rises in Newark, N.J., Dec. 13, 2021.
Gary Hershorn/Getty Images
A United Airlines airplane is pushed back from its gate at Newark Liberty International Airport as the sun rises in Newark, N.J., Dec. 13, 2021.

And it's not just Delta that's feeling the impact of the variant on crews.

As of Saturday afternoon, United had canceled 240 Christmas Day flights and 85 flights scheduled for Sunday. It had canceled 201 flights on Christmas Eve.

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

"We're sorry for the disruption and are working hard to rebook as many people as possible and get them on their way for the holidays," the airline added.

JetBlue has canceled 123 Christmas Day flights due to COVID issues. It had canceled 80 flights Christmas Eve.

"Like many businesses and organizations, we have seen an increasing number of sick calls from Omicron," JetBlue said in a statement Friday. "We entered the holiday season with the highest staffing levels we've had since the pandemic began and are using all resources available to us to cover our staffing needs. Despite our best efforts, we've had to cancel a number of flights, and additional flight cancellations and other delays remain a possibility as we see more Omicron community spread."

Alaska Airlines has resorted to offering extra pay to their healthy employees who can work added shifts into this upcoming Christmas weekend.

The airline says they have had to cancel 10 Christmas Eve flights due to some of their employees quarantining after reporting that they may have been exposed to COVID-19.

PHOTO: People wait in line to check in at the United Airlines ticket counter at Hartsfield-Jackson Atlanta International Airport in Atlanta, Dec. 20, 2021.
Elijah Nouvelage/Reuters
People wait in line to check in at the United Airlines ticket counter at Hartsfield-Jackson Atlanta International Airport in Atlanta, Dec. 20, 2021.

American Airlines canceled 88 flights on Christmas Day.

"Our operation has been running smoothly, and unfortunately a number of COVID-related sick calls led us to make the difficult decision to precancel some flights scheduled for today," a representative for American Airlines said in a statement to ABC News on Saturday. "We proactively notified affected customers yesterday, and are working hard to rebook them quickly. We never want to disappoint our customers and apologize for any disruptions to their holiday travel plans."

Airlines for America, the group that lobbies on behalf of all major U.S. airlines, is calling on the Centers for Disease Control and Prevention to shorten the quarantine time for fully vaccinated individuals, saying the omicron surge may create "significant" disruptions.

"The omicron surge may exacerbate personnel shortages and create significant disruptions to our workforce and operations," Nick Calio, A4A's CEO, said in a letter on Thursday to CDC Director Rochelle Walensky.

Calio proposed the isolation period to be shortened to five days from symptom onset for breakthrough infections.

"In turn, those individuals would be able to end isolation with an appropriate testing protocol," Calio wrote.

The letter comes after Delta Air Lines and JetBlue Airways, both A4A members, also asked for isolation periods for fully vaccinated individuals to be shortened.