U.S. warship sails within 12 miles of Chinese-built island in South China Sea

The USS Lassen, a guided-missile destroyer, was accompanied by Navy surveillance planes as it approached the Subi Reef in the Spratly Islands, the official said.

The mission "was completed without incident," said the official, who spoke on the condition of anonymity.

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The decision to go ahead with the mission follows months of deliberation in Washington and is certain to anger China, which said last month it would "never allow any country" to violate its territorial waters and airspace around the islands.

Chinese Foreign Minister Wang Yi had said earlier that Beijing was trying to verify whether the U.S. vessel had entered the 12-mile zone.

"If true, we advise the U.S. to think again, not to act blindly or make trouble out of nothing," the Foreign Ministry quoted him as saying.

The U.S. mission is also intended to test a pledge made by President Xi Jinping during his visit to Washington last month that Beijing would not militarize the islands, U.S. officials have said.

Subi Reef, which lies close to the Philippines in the South China Sea, used to be submerged at high tide before China began a massive dredging project to turn it into an island. It is now big enough to potentially host an airstrip.

Satellite images also show what looks like a surveillance tower and multiple satellite antennas, according to the Asia Maritime Transparency Initiative, part of the Center for Strategic and International Studies.

Under the international law of the sea, turning such features into artificial islands does not imply any rights to territorial waters around them, something the U.S. mission is designed to underline.

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

"This should have been done a long time ago," said Bonnie Glaser, an expert on the Chinese military at the Center for Strategic and International Studies. The Navy has wanted conduct such an operation for some time, but the Obama administration had prevented that until now, she said.

While the exercise would probably not stop China from further construction or militarization of air strips on the reefs, Glaser said that there were still good reasons to go ahead.

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"Our aim in any freedom of navigation operation is not aimed at that kind of objective. It is simply to sail through waters that are subject to the law of the sea," she said.

"Some parts of the administration believed this would make things even more difficult, that China would become even more obstreperous, more difficult to deal with," she said, "and others thought this wasn't something we should do before Xi Jinping came to Washington."

At the summit, President Barack Obama told Xi that the United States would operate, fly or sail anywhere that international law allows. On Monday, White House spokesman Josh Earnest referred questions on specific operations to the Pentagon but reiterated that commitment to freedom of navigation.

"This is a critically important principle, particularly in the South China Sea, because there are billions of dollars of commerce that flow through that region of the world every year, and maybe even more than that and ensuring the free flow of this commerce and that freedom of navigation of those vessels is protected is critically important to the global economy."

Additional patrols will follow in coming weeks, and could also be conducted around features that have been built up by Vietnam and the Philippines in the Spratlys, a U.S. defense official told the Reuters news agency.

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

At the U.S. Naval War College in Newport, R.I., associate professor Andrew Erickson said the U.S action underscored its "commitment to maintaining an open global system with global commons that all are free to use to the maximum extent permitted by international law."

"As can be seen from the operation's peaceful, unimpeded nature, China and the U.S. share an interest in keeping the vital sea lanes of the South China Sea stable and open," he said.

China has built small islands on top of previously submerged reefs like Subi and is in the process of smoothing and reinforcing runways suitable for military aircraft. According to international law, experts said no country can claim more than a 500-meter safety zone around a previously submerged reef.

The USS Lassen, an Arleigh Burke-class guided-missile destroyer, operates out of its home port in Yokosuka, Japan - headquarters for the U.S. 7th Fleet.