Two U.S. B-52 strategic bombers flew near artificial Chinese islands in the South China Sea this week, the Pentagon said Thursday, calling the planes' appearance routine.

The mission followed a patrol last month by a U.S. guided-missile destroyer, which passed within waters that Beijing claims.

During the air mission, Chinese ground controllers contacted the pilots as they passed, but the nuclear-capable jets continued their mission undeterred.

China claims most of the South China Sea, through which more than $5 trillion in global trade passes every year, and the United States has said it will continue conducting patrols to assure freedom of navigation. Vietnam, Malaysia, Brunei, the Philippines and Taiwan have rival claims in the region.

In the latest mission, which occurred overnight on Nov. 8-9, the bombers flew "in the area" of the Spratly Islands but did not come within the 12-nautical-mile (22-km) zones that China claims as territory around islands it has built in the chain, said Commander Bill Urban, a Pentagon spokesman.

"The B-52s were on a routine mission in the (South China Sea)," taking off from and returning to Guam, Urban said.

Chinese ground controllers contacted the bombers but the aircraft continued their mission unabated, Urban said.

"We conduct B-52 flights in international air space in that part of the world all the time," a Pentagon spokesman told a news briefing earlier on Thursday.

Last month, a U.S. warship challenged territorial limits around one of China's man-made islands in the Spratly archipelago with a so-called freedom-of-navigation patrol, the most significant U.S. challenge yet to territorial limits China claims around its new islands.

U.S. and Chinese warships and aircraft regularly communicate when operating near each other in Asian waters, U.S. military officials say.