China has lodged "solemn" representations with the U.S. after an American destroyer continuously conducted close reconnaissance of normal exercises and training of China's Liaoning aircraft carrier fleets in waters near China, Wu Qian, spokesperson for the Ministry of National Defense, told a news conference on Thursday.

The Chinese navy warned and expelled the destroyer USS Mustin, Wu said. 

"The (U.S.) action seriously threatens the safety of ships and personnel of both Chinese and U.S. sides. The U.S. vessel was later urged to leave," he said.

Wu made the remarks in response to a query about frequent activities of U.S. warships and aircraft in waters and airspace near China.

U.S. warships have been 20 percent more active in sea areas around China compared with the same period last year, and reconnaissance aircraft have been 40 percent more active since the current U.S. administration took office, he said.

The Chinese side, he continued, firmly opposes such actions as they escalate militarization in the region and threaten its peace and stability.

"We urge the U.S. side to strictly restrain its frontline troops and comply with regulations, such as the Rules of Behavior for the Safety of Air and Maritime Encounters and the international regulations for preventing collisions at sea, so that such dangerous incidents do not reoccur," Wu said.

(With input from Xinhua)