WASHINGTON, Nov 12 - Two U.S. B-52 strategic bombers flew near artificial Chinese-built islands in the East Vietnam Sea this week and were contacted by Chinese ground controllers but continued their mission undeterred, the Pentagon said on Thursday.

The latest U.S. patrol in the disputed East Vietnam Sea occurred in advance of President Barack Obama's visit to the region next week to attend Asia-Pacific summits where he is expected to reassert Washington's commitment to freedom of navigation and overflight in the area.

China claims most of the East Vietnam 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 unimpeded passage. Vietnam, Malaysia, Brunei, the Philippines and Taiwan have rival claims in the region.

In the latest mission, on the night of Nov. 8 and 9, the bombers flew "in the area" of Vietnam’s Truong Sa (Spratly) archipelago but did not come within the 12-nautical-mile 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 East Vietnam 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," Pentagon spokesman Peter Cook told a news briefing on Thursday.

In Beijing, asked about the case, Chinese Foreign Ministry spokesman Hong Lei said China respected all countries exercising freedom of navigation and overflight in the East Vietnam Sea in line with international law.

"We resolutely oppose any country, in the name of freedom of navigation and overflight, harming and violating international law, harming China's sovereignty and security interests," he added.

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

China reacted angrily to the patrol.

White House spokesman Josh Earnest said he did not know whether the East Vietnam Sea would be on the formal agenda at any of the three Asia summits Obama is to attend but added it would be "on the minds and lips" of the gathered world leaders.

Obama's first stop will be Manila, for the Asia-Pacific Economic Cooperation forum summit, at which Chinese President Xi Jinping will also be present. The U.S. president will then go to Kuala Lumpur for ASEAN and East Asia summits.

"We are quite concerned about protecting freedom of navigation, the free flow of commerce in the [East Vietnam Sea]," Earnest told reporters.

"And we're going to continue to encourage all parties, big and small, to resolve their differences diplomatically and to not try to use their comparative size and strength to intimidate their neighbors."

Chinese spokesman Hong said the East Vietnam Sea should not be a subject for discussion at the East Asia Summit.

"The East Asia Summit and relevant meetings focus on regional cooperation and development," he said. "They are not an appropriate place for discussing the [East Vietnam Sea] issue."

In an apparent show of U.S. resolve, Obama will take part in what the White House called "an event that showcases U.S. maritime security assistance to the Philippines." U.S. officials did not elaborate.

But in September, Navy Admiral Harry Harris, head of the U.S. Pacific Command, visited the National Coast Watch Center, a facility at the Philippines coast guard headquarters that Washington has helped Manila build, to better monitor developments in the East Vietnam Sea.