American Airlines Group Inc. and United Airlines Holdings Inc. said Chinese aviation regulators have blocked some of their flights from entry as the country steps up precautions against Covid-19.

At least six of American’s flights have been barred from China, a spokesperson said Tuesday, declining to provide further details. United canceled flights from San Francisco to Shanghai on six days in late January. Delta Air Lines Inc. didn’t immediately comment on its service to China.

The flights are among 60 between the two countries that the Civil Aviation Administration of China has ordered canceled after passengers tested positive for Covid-19 after arriving in the country, Reuters reported, including 22 by U.S. carriers. The action is the latest scuffle between the U.S. and China over flights between the countries and requirements to help contain the spread of coronavirus.

United normally has four weekly flights from San Francisco to Shanghai, while American operates to the Chinese city from Dallas-Fort Worth twice a week.

Hong Kong is set to bar air passengers from designated high-risk countries from traveling through Hong Kong International Airport, people familiar with the matter said. It’s the latest move in the financial hub’s aggressive effort to shield itself from the omicron coronavirus variant.