WASHINGTON (AP) -- Large U.S. airlines and some of their unions are asking the Biden administration to stop approving any more flights between the United States and China because of what they call "anti-competitive" policies that China imposes on U.S. carriers.

The airlines and unions said Thursday that China closed its market to U.S. carriers at the outbreak of the pandemic and imposed rules that still affect American operations and airline crews.

"These actions demonstrated the clear need for the U.S. government to establish a policy that protects U.S. aviation workers, industry and air travelers," they said in a letter to Secretary of State Antony Blinken and Transportation Secretary Pete Buttigieg.

The letter was signed by the CEO of the Airlines for America trade group and the presidents of the Air Line Pilots Association, the Allied Pilots Association, which represents crews at American Airlines, and the Association of Flight Attendants.

The number of flights between China and the U.S. has been rising, although it remains far below pre-pandemic levels. The Biden administration increased the number of round trips that Chinese airlines can make from 35 to 50 per week, starting March 31, after China's aviation authority promised to seek an increase in flights by U.S. carriers.

The U.S. airlines said Chinese airlines get an advantage by flying shorter routes through Russian airspace, which has been off-limits to U.S. carriers since Russia invaded Ukraine more than two years ago. They said Chinese airlines also get "certain protections" from China's government because they are state-owned.

The U.S. industry groups said in their letter that without equal access to China's aviation market, American carriers will lose flights to Chinese airlines.