More than 3,000 flights scheduled for Christmas Eve and Christmas Day have been canceled globally, according to the website FlightAware.
Around 20 percent of those canceled for Friday involved travel within, into or out of the United States, FlightAware said.
So far, 4,604 flights have been delayed. About 500 flights are delayed within, into, or out of the United States on Friday.
United Airlines and Delta Air Lines said Thursday that they had canceled hundreds of Christmas Eve flights.
United Airlines confirmed that more than 100 flights that were scheduled for Christmas Eve have been canceled by the company as it grapples with the spread of the omicron variant.
A spokesperson for United said on Thursday that "we’re seeing about 120 cancellations planned for Friday."
"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.
"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,” United added.
Delta said Thursday it has "exhausted all options and resources - including rerouting and substitutions of aircraft and crews to cover scheduled flying - before canceling around 90 flights for Friday."
JetBlue has also canceled more than 50 flights. Alaska Airlines said in a statement to CNN that it had canceled 17 flights on Thursday because of omicron, and that more cancellations were possible on Friday. Per CNN, it has cancelled 11 more flights as of Friday morning.
White House chief medical adviser Anthony FauciAnthony FauciOvernight Health Care — ObamaCare gets record numbers Fans attending Super Bowl LVI to be given KN95 masks The Hill's 12:30 Report - Presented by Facebook - Breaking: Justice Breyer to retire MORE said earlier this week that large gatherings this holiday season are not safe from the coronavirus, even for those fully vaccinated with a booster shot.
He also warned that that Christmas travel would increase the spread of the variant even among the fully vaccinated, the BBC reported.
David Powell, physician and medical adviser to the International Air Transport Association, a group representing nearly 300 airlines, said in a new interview that aircraft passengers are two or three times more likely to be infected with COVID-19 on a flight since the emergence of the omicron variant.
"Whatever the risk was with delta, we would have to assume the risk would be two to three times greater with omicron, just as we’ve seen in other environments," Powell told Bloomberg News.
The flight cancellations come as Francis Collins, the outgoing National Institutes of Health director, warned on Sunday that the U.S. could begin seeing 1 million COVID-19 infections per day if Americans don’t take coronavirus precautions.
The retiring NIH director, whose last day was Sunday, cautioned on NPR's "Weekend Edition" that the highly transmissible omicron variant could still spark millions of infections in the U.S., even if it causes milder cases of the virus than other strains, as early studies have suggested.
--Updated on Dec. 25 at 7:30 a.m.