The global tally exceeded 3,500 trips for Friday and Saturday, according to data tracker FlightAware.com. China Eastern and Air China ranked one-two, and the most-affected airport was in the Chinese city of Xi’an, where the Beijing government cracked down under its Covid Zero policy after an outbreak.

United set the U.S. cancellations in motion late Thursday, and its count swelled overnight into Friday, with 280 flights erased on Christmas Eve and Christmas Day. FlightAware said Delta cut 278 flights. JetBlue Airways Corp. chopped 72 flights, or 7% of its Friday schedule, according to the website. 

The travel snarls underscored the reach of the omicron Covid variant that is driving U.S. case counts higher, increasing nearly sixfold in only a week to become the dominant domestic strain. United said in an email Thursday that a jump in omicron was limiting the availability of flight crews and ground personnel. Delta attributed its disruptions to a combination of reasons. 

“As winter weather impacts the northwest and northeast U.S. and the omicron variant continues to surge, Delta teams exhausted all options and resources” before turning to cancellations, Delta said in an emailed statement Friday. 

The holidays are among the heaviest times for travel. The Transportation Security Administration was estimating Dec. 22 and 23 would be the busiest pre-Christmas travel dates nationally and locally, with Jan. 2 and 3 the most crowded for post-holiday travel.

“We’ve unfortunately had to cancel some flights and are notifying impacted customers in advance of them coming to the airport,” United said. “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.”

JetBlue didn’t immidiately respond to emailed questions about the cause of its cancellations. 

Weather compounded airlines’ woes at some crucial U.S. airports. Chicago’s O’Hare International Airport, a hub for United and American Airlines, had the most tardy departures in the U.S. at 65. Mist and low clouds limited visibility to less than 2 miles (3.2 kilometers). 

Globally, cancellations ran highest at Xi’an Xianyang International Airport, in the western Chinese city where the government punished local officials for failing to curb an outbreak that led to the biggest lockdown since COVID emerged in Wuhan. Nearly a third of the flights departing from the airport were canceled Friday, while about 25% were canceled on Christmas Day, according to FlightAware. 

Elsewhere, British discounter EasyJet said it had canceled some flights between the U.K. and France and Germany for the rest of December, following the introduction of restrictions on U.K. travelers to those countries.

(Updates throughout with new flight totals. With assistance from Charlotte Ryan and Maria Elena Vizcaino.)