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, triggering discussions between industry representatives and government agencies, according to China Travel News.

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. declined to comment.

The flights are among 60 between the two countries that the Civil Aviation Administration of China has ordered canceled because 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 changing requirements to help contain the spread of coronavirus.