Delta Air Lines on Monday cited new Chinese cleaning requirements after it rerouted a China-bound flight back to the United States, drawing criticism from Chinese authorities.

The December 21 flight was supposed to land in Shanghai after disembarking from Seattle, but instead returned to the US city midair. Delta said the reason for the reversal was new Chinese requirements issued earlier that day in the wake of the latest Covid-19 surge.

“The new cleaning procedures require significantly extended ground time and are not operationally viable for Delta,” a Delta spokesman said. “We apologize for any inconvenience this is causing for customers as we continue to work on rebooking on alternate flights.”

The Chinese consulate in San Francisco complained that several Chinese citizens were stranded on board with expired visas and Covid-19 tests that no longer met time requirements, according to Chinese state media.

Airlines, including Delta and other leading US carriers, have canceled thousands of flights in recent days as the Omicron variant of Covid-19 hits airline workers and roils travel plans for many consumers.

China, like other countries, is battling the latest resurgence of the virus.

On Monday, Chinese city Xi’an tightened Covid-19 controls to the “strictest” level, banning residents from driving cars around town in an effort to control the country’s worst outbreak in 21 months.