Delta Air Lines blames new cleaning rules in Shanghai for the midair turnaround of a flight from Seattle.

A Delta Air Lines flight that was en route from Seattle to Shanghai turned back in midair last week because the Chinese airport had imposed new cleaning rules that would have caused delays, the airline said in a statement on Tuesday.

“The new cleaning procedures require significantly extended ground time and are not operationally viable for Delta,” the airline said.

Details of the new cleaning regulations in place at Shanghai Pudong International Airport were not immediately clear, but China has rolled out increasingly strict Covid-19 travel rules amid a growing outbreak in the northern city of Xian and before the Winter Olympics in Beijing in February.

The scrapped flight left passengers with out-of-date Covid-19 test results and expired U.S. visas, according to Chinese news reports. The Chinese Consulate in San Francisco, which did not name Delta, said in a statement that it “had made a stern representation to the airline.”

The new sanitation protocols also led EVA Air of Taiwan to suspend passenger services to Shanghai Pudong from the cities of Taipei and Kaohsiung until Feb. 3, Taiwan’s semiofficial Central News Agency reported. Another Taiwan-based carrier, China Airlines, is suspending flights from one city to Shanghai until the end of January and reducing flights on another route, according to news reports.

The Chinese Embassy in the United States said in a statement that reports that the Delta plane had been turned around because of a ban on incoming flights were incorrect. Many domestic and international flights in the United States had been canceled because of staff shortages, the embassy noted, adding that it was “communicating with relevant airlines to actively understand the specific technical issues and discuss solutions to avoid similar incidents from happening again.”

The outbreak in Xian remains small by global standards. On Tuesday, Global Times, a Communist Party tabloid in China, said that the city of 13 million people, where the local authorities last week imposed a lockdown, had reported 175 cases in one day, its highest daily count since the outbreak began this month.

In total, 810 cases have been recorded in Xian.