Defense Ministry spokesman Ren Guoqiang also reiterated at a monthly briefing China's objections to a recent U.S. arms sale to Taiwan and the imposition of U.S. sanctions over China's purchase of Russian defense equipment.

"As for the provocative action taken by the U.S. military aircraft, we are firmly against it and we will take all necessary means to safeguard our rights and interests," Ren said.

Two B-52s flew over the strategic waterway, largely claimed by China, earlier this week in what the Pentagon called a routine mission. Separately, two B-52s also flew this week over the East China Sea, where China has declared an air defense identification zone and claims uninhabited islands controlled by Japan.

China has sought to strengthen its claim to the South China Sea by building seven islands on reefs and equipping them with military facilities such as airstrips, radar domes and missile systems. Five other governments claim territory in the oil- and gas-rich area through which an estimated $5 trillion in global trade passes annually.

Asked at the Pentagon on Wednesday about the bomber flights, Defense Secretary Jim Mattis said he wasn't concerned they might raise tensions with Beijing.