TOKYO, Oct 27 - The U.S. Navy sent a warship on Tuesday within 12 nautical miles of two artificial islands built by China in the East Vietnam Sea, a U.S. defence official told Reuters, in a challenge to China's territorial claims in the area.

The official said the patrol was carried out by the destroyer USS Lassen near Da Xu Bi (Subi) and Da Vanh Khan (Mischief) reefs in Vietnam’s Truong Sa (Spratly) archipelago, features that were formerly submerged at high tide before China began a massive dredging project to turn them into islands in 2014.

"The operation has begun ... It will be complete within a few hours," said the official.

Earlier, the official said the ship would likely be accompanied by a U.S. Navy P-8A surveillance plane and possibly P-3 surveillance plane, which have been conducting regular surveillance missions in the region.

The patrols represent the most serious U.S. challenge yet to the 12-nautical-mile territorial limit China claims around the islands and are certain to anger Beijing, which said last month it would "never allow any country" to violate its territorial waters and airspace in Truong Sa.

The Chinese Embassy in Washington said the concept of freedom of navigation should not be used as an excuse for muscle flexing and the United States should "refrain from saying or doing anything provocative and act responsibly in maintaining regional peace and stability."

Additional patrols would follow in coming weeks and could also be conducted around features that Vietnam and the Philippines have built up in the archipelago, the U.S. official said.

"This is something that will be a regular occurrence, not a one-off event," said the official. "It's not something that's unique to China."

White House spokesman Josh Earnest referred questions on any specific operations to the Pentagon but said the United States had made clear to China the importance of free flow of commerce in the East Vietnam Sea.

"There are billions of dollars of commerce that float through that region of the world," Earnest told a news briefing. "Ensuring that free flow of commerce ... is critical to the global economy," he said.

The patrols would be the first within 12 miles of the features since China began building the reefs up in 2014. The United States last went within 12 miles of Chinese-claimed territory in Truong Sa in 2012.

The decision to go ahead with the patrols follows months of deliberation and risks significantly upsetting already strained ties with China, the world's second-biggest economy, with which U.S. business and economic interests are deeply intertwined.

U.S. Congressman Randy Forbes, chairman of the House Armed Services Seapower and Projection Forces Subcommittee and co-chairman of the congressional China Caucus, praised the plan.

The passage of U.S. vessels within 12 nautical miles of China's man-made features in the East Vietnam Sea is “a necessary and overdue response to China's destabilizing behavior in the region," Forbes said.

China claims most of the East Vietnam Sea, one of the world's busiest sea lanes, through which more than $5 trillion of world trade passes every year. Vietnam, Malaysia, Brunei, the Philippines and Taiwan have rival claims.

Competing claims

The United States argues that under international law, building up artificial islands on previously submerged reefs does not entitle a country to claim a territorial limit and that it is vital to maintain freedom of navigation.

Washington worries that China has built up the islands with the aim of extending its military reach in the East Vietnam Sea.

The patrols would be just weeks ahead of a series of Asia-Pacific summits President Barack Obama and Chinese President Xi Jinping are expected to attend in the second half of November.

Xi surprised U.S. officials after a meeting with Obama in Washington last month by saying that China had "no intention to militarize" the islands.

Even before that, however, satellite photographs had shown the construction of three military-length airstrips by China in Truong Sa, including one each on Da Vanh Khan (Mischief) and Da Xu Bi (Subi) reefs.

Some U.S. officials have said that the plan for patrols was aimed in part at testing Xi's statement on militarization.

In May, the Chinese navy issued eight warnings to the crew of a U.S. P8-A Poseidon surveillance aircraft that flew near the artificial islands but not within the 12-mile limit, reported CNN, which was aboard the U.S. aircraft.

That same month, the USS Fort Worth, a littoral combat ship, "encountered multiple" Chinese warships during a patrol in Truong Sa, the U.S. Navy said at the time. It did not go into detail.

In 2013, Obama ordered two B-52 bombers to fly through an Air Defense Identification Zone that China established in the East China Sea over territory contested with Japan.

Pentagon officials say the United States regularly conducts freedom-of-navigation operations around the world to challenge excessive maritime claims.

In early September, China sent naval vessels within 12 miles of the Aleutian Islands off Alaska. China said they were there as part of a routine drill following exercises with Russia.