China has refused port visits to Hong Kong by two U.S. warships amid continued trade tensions and diplomatic spats between the two sides over pro-democracy protests in the Asian financial hub.

The Chinese government denied permission for the USS Green Bay and the USS Lake Erie to visit Hong Kong in the coming weeks, Cmdr. Nate Christensen, a deputy spokesman for the U.S. Pacific Fleet, said. The Green Bay is an amphibious transport dock while the Lake Erie is guided-missile cruiser.

“The U.S. Navy has a long track record of successful port visits to Hong Kong, and we expect them to continue,” Christensen said. “We refer you to the Chinese government for further information about why they denied the request.”

China’s foreign ministry said in a statement Wednesday that the country has always approved port visits by U.S. warships “case by case, on the basis of sovereignty principles and specific situations,” without elaborating.

The move comes as the U.S. and China spar in a protracted trade war and American lawmakers criticize the Hong Kong police’s tactics against demonstrators who have protested for more than two months. China has accused the U.S. of instigating protesters to violence, citing communications between American officials and activists — a claim Washington denies.

President Donald Trump said in a tweet Tuesday that reports from U.S. intelligence agencies show the Chinese government is moving troops to its border with Hong Kong, stoking fears of a possible intervention. “Everyone should be calm and safe!” Trump said, without providing details about when he received the information.

Beijing last refused a U.S. naval port call to the former British colony in September, when it denied a visit to the amphibious assault ship the USS Wasp days after Washington sanction the Chinese military over Russian arms purchases.