By Andrea Shalal and David Brunnstrom

WASHINGTON (Reuters) -- The U.S. Navy plans to send the destroyer USS Lassen within 12 nautical miles of artificial islands built by China in the South China Sea within 24 hours, in the first of a series of challenges to China's territorial claims, a U.S. defense official said on Monday.

The patrol would occur near Subi and Mischief reefs in the 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 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, according to the official, who spoke on condition of anonymity.

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

“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. "Ensuring that free flow of commerce ... is critical to the global economy," he said.

The patrols will mark the most serious U.S. challenge yet to the 12-nautical-mile territorial limit China claims around the islands and follows months of deliberation.

The move 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.

China claims most of the South China Sea and on Oct. 9 its Foreign Ministry warned that Beijing would "never allow any country to violate China's territorial waters and airspace in the Spratly Islands, in the name of protecting freedom of navigation and overflight."

It would be the first time the United States has gone within 12 miles of the features since China began building the reefs up in 2014. It last went within 12 miles of Chinese-claimed territory in the Spratlys in 2012.


The patrols will come 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.

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 in a sea through which more than $5 trillion of world trade passes every year.

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

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.

However, even before that, satellite photographs had shown the construction of three military-length airstrips by China in the Spratlys, including one each on Mischief and Subi reefs.

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

China has overlapping claims in the South China Sea with Vietnam, Malaysia, Brunei and the Philippines, all of which are members of the Association of Southeast Asian Nations (ASEAN), as well as with Taiwan.

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, according to 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 the Spratly archipelago, 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.

(Reporting by Andrea Shalal and David Brunnstrom; Editing by Sandra Maler and Cyntha Osterman)