WASHINGTON/BEIJING, Oct 27 (Reuters) - The U.S. Navy sent a guided-missile destroyer close to China's man-made islands in the disputed South China Sea on Tuesday, U.S. defense officials said, a move that could heighten tensions over the strategic waterway.
The patrol represents the most serious U.S. challenge to 12-nautical-mile territorial limits China asserts around the islands and is sure to anger Beijing, which said last month it would never allow any country to violate its territory in the Spratly archipelago, where seven outposts have been built.
One U.S. defense official said the USS Lassen sailed within 12 nautical miles of Subi Reef. A second official said the patrol, which lasted a few hours, also included Mischief Reef.
The second official said the mission would be the start of a series of challenges to China's territorial claims in one of the world's busiest sea lanes.
China's Foreign Minister Wang Yi said Beijing was trying to verify if the U.S. ship had entered the 12-mile zone.
"If true, we advise the U.S. to think twice before acting, not act blindly or make trouble out of nothing," the Foreign Ministry quoted Wang as saying.
The Chinese Embassy in Washington said the concept of freedom of navigation should not be used as an excuse for muscle flexing.
The second U.S. official earlier said the ship would likely be accompanied by a U.S. Navy P-8A surveillance plane and possibly a P-3 surveillance plane, which have been conducting regular missions in the region.
Additional patrols would follow in the coming weeks and could also be conducted around features that Vietnam and the Philippines have built up in the Spratlys, the 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 South China Sea.
"There are billions of dollars of commerce that float through that region of the world," Earnest told a news briefing.
The United States had not conducted a patrol within 12 miles of the features since China began building the reefs up at the end of 2013. The U.S. Navy last went within 12 miles of Chinese-claimed territory in the Spratlys in 2012.
The decision to go ahead follows months of deliberation and risks upsetting already strained ties with China, the world's second-biggest economy, with which U.S. business and economic interests are deeply intertwined.
China claims most of the South China Sea, through which more than $5 trillion of world trade passes every year. Vietnam, Malaysia, Brunei, the Philippines and Taiwan have rival claims.
Euan Graham, director of the International Security Program at the Lowy Institute in Sydney, said while there was likely to be a strong vocal reaction from China, its military response could be muted.
"I would expect it would be diplomatically shrill in its reaction. China has also in some ways backed itself into a corner by saying it will exert its sovereignty over the Spratlys," Graham said.
The patrol could prompt China to do more to exert its sovereignty in the region through further reclamations and greater militarization, he added.
Both Subi and Mischief Reefs were submerged at high tide before China began a massive dredging project to turn them into islands in 2014.
Under the U.N. Convention on the Law of the Sea, 12-nautical mile limits cannot be set around man-made islands built on previously submerged reefs.
Washington worries that China has built up its outposts with the aim of extending its military reach in the South China Sea. China says they will have mainly civilian uses as well as undefined defense purposes.
The patrol comes 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 the Spratlys, including one each on Subi and Mischief 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.
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.
(Reporting by Andrea Shalal, Yeganeh Torbati and David Brunnstrom in Washington and Ben Blanchard in Beijing; Additional reporting by Tim Kelly in Tokyo and Lincoln Feast in Sydney; Editing by Dean Yates and Alex Richardson)