The Trump administration moved Wednesday to block Chinese airlines from flying to the U.S. in an escalation of trade and diplomatic tensions between the two countries.

The Transportation Department said it would suspend passenger flights of four Chinese airlines to and from the United States starting June 16.

The decision was in response to China's failure to let United Airlines and Delta Air Lines resume flights to China this month. The airlines suspended those flights earlier this year in response to the coronavirus pandemic that started in China's Wuhan province.

On Thursday, the Chinese air regulator said more airlines would be allowed to resume flights to and from China but gave no indication whether United and Delta were included.

An employee who answered the phone at the Civil Aviation Administration of China and would give only her surname, Yan, said she had no details on the status of United and Delta.

The Transportation Department said that China was violating a 1980 agreement between the two countries covering flights by each other's airlines. The department said it would continue talks with Chinese officials to settle the dispute.

"In the meantime, we will allow Chinese carriers to operate the same number of scheduled passenger flights as the Chinese government allows ours," the Transportation Department said in a statement.

The department said U.S. President Donald Trump could put the order into effect before June 16. The administration had hinted at Wednesday's move last month, when it protested to Chinese authorities that Beijing was preventing U.S. airlines from competing fairly against Chinese carriers.

The four airlines affected by the order are Air China, China Eastern Airlines, China Southern Airlines and Xiamen Airlines.

Before the pandemic, there were about 325 passenger flights a week between the United States and China, including ones operated by United, Delta and American Airlines. While U.S. carriers stopped their flights, Chinese airlines continued to fly scaled-down schedules between the two countries -- 20 flights a week in mid-February and 34 a week by mid-March.

To curb the spread of coronavirus, China limited foreign airlines to one flight per week based on schedules that they operated in mid-March. Since U.S. airlines had already stopped flying to China by then, that effectively has shut them out, the Transportation Department said.

The department said it objected, but China's aviation agency said last week it was not violating the air-travel treaty because the same one-flight limit applies to Chinese airlines.

Foreign airlines that aren't on the March list will be allowed to make one flight per week to a port city that has the capability to receive them and is within the scope of their license, CAAC said in a written statement.

Carriers will be allowed to increase that to two flights per week if they go three weeks with no passengers testing positive for the virus, CAAC said. It said a route will be suspended for one week if the number of passengers who test positive reaches five.

United and Delta announced last month that they hoped to resume flights to China in June, as air travel has begun to recover recently. United wants to fly from San Francisco to Shanghai and Beijing and from Newark, New Jersey, to Shanghai. Delta seeks to resume flights via Seoul to Shanghai from Seattle and Detroit.

"We support and appreciate the U.S. government's actions to enforce our rights and ensure fairness," Delta spokeswoman Lisa Hanna said.

United Airlines spokesman Frank Benenati said, "We look forward to resuming passenger service between the United States and China when the regulatory environment allows us to do so."

Messages to a spokesperson in China's embassy in Washington were not immediately answered, and efforts to reach the person by phone were unsuccessful.

Eswar Prasad, a professor of trade policy at Cornell University, said the back and forth will increase political tensions between the U.S. and China, "which already seem to have passed a point of no return."

But Jeff Moon, a former State Department official and now a trade consultant, said the airline dispute was less complicated than other conflicts between the two countries.

"This case can be resolved if cooler heads prevail ... and there is a genuine desire to restore air links," he said.

As the administration moved against the airlines, it also stepped up its criticism of China on the 31st anniversary of the deadly crackdown on pro-democracy protesters in Beijing's Tiananmen Square.

Secretary of State Mike Pompeo tweeted out a photograph of him meeting Tuesday with several survivors of the crackdown. Shortly after that, the State Department released a statement saying Pompeo was honoured to meet the four dissidents, whom it called "brave participants in the heroic protests for democracy that were brutally put down by the Chinese Communist Party on June 4, 1989."

Chinese Foreign Ministry spokesman Zhao Lijian responded, taking aim at the U.S. over civil unrest over police killings of blacks. The protests "once again reflect the racial discrimination in the U.S., the serious problems of police violent enforcement and the urgency of solving these problems," Zhao said.

------

Matthew Lee in Washington and AP researcher Yu Bing in Beijing contributed.