China said it was “very displeased” after a U.S. Navy destroyer conducted a so-called freedom of navigation patrol (FONOP) within 12 nautical miles (22 km) of an artificial island built up by China in the South China Sea.

The operation, which involved the Yokosuka, Kanagawa Prefecture-based USS John S. McCain guided-missile destroyer, was conducted Thursday around Mischief Reef in the Spratly chain of the strategic waterway, and was first reported by Reuters, citing anonymous U.S. officials.

China’s Foreign Ministry confirmed the patrol, saying that the operation had violated international and Chinese law and seriously harmed Beijing’s sovereignty and security, adding that the Chinese Navy had “warned and expelled” the U.S. warship.

“The Chinese side is very displeased and will make solemn representations to the U.S.,” the ministry said in a statement.

The country’s Defense Ministry also lambasted the U.S. move, calling it a “provocative act” that had seriously damaged mutual trust.

“China is resolutely opposed to this kind of show of force and pushing of regional militarization by the U.S. that may easily cause an unexpected incident at sea or in the air,” the ministry said in a statement.

“U.S. military provocations of this kind will only encourage the Chinese military to further strengthen defense-capacity building,” it added

Asked to confirm or deny the operation, a Pentagon spokesman told the The Japan Times only that the United States is “continuing regular FONOPs.”

“U.S. forces operate in the Asia-Pacific region on a daily basis, including in the South China Sea,” spokesman Lt. Col. Christopher Logan said. “All operations are conducted in accordance with international law and demonstrate that the United States will fly, sail, and operate wherever international law allows.

“We are continuing regular FONOPs, as we have routinely done in the past and will continue to do in the future,” he added. “Summaries of these operations will be released publicly in the annual FONOPS report, and not sooner.”

The U.S. Defense Department says FONOPs represent “a challenge to excessive maritime claims.” The significance of the distance of 12 nautical miles derives from the U.N. Convention on the Law of the Sea (UNCLOS), which generally grants coastal states jurisdiction over seas within 12 nautical miles of the coast.

Thursday’s operation came as U.S. President Donald Trump seeks Chinese cooperation in reining in North Korea’s missile and nuclear programs, and could complicate efforts to work together toward that end.

The patrol was the third under Trump, and the second this year around Mischief Reef, a land feature that was included in a ruling last year against China by an international arbitration court in The Hague. The ruling invalidated Beijing’s claim to sovereignty over large swaths of the South China Sea.

China, which despite a pledge to the contrary, has continued to militarize the waters as it seeks to reinforce effective control of much of the waterway, through which at least $3 trillion in trade passes each year. The Philippines, Vietnam, Malaysia, Taiwan and Brunei also have overlapping claims.

China has built seven man-made islets in the South China Sea’s hotly contested Spratly chain, with three boasting military-grade airfields — including Mischief Reef — despite a 2015 vow by Chinese President Xi Jinping not to further “militarize” them. Beijing has called the moves purely defensive.

The Center for Strategic and International Studies’ Asia Maritime Transparency Initiative (AMTI) wrote in an analysis just after November’s U.S. presidential election that the FONOP location most directly related to enforcement of the arbitration court ruling was Mischief Reef, which was found to be a low-tide elevation that does not generate an entitlement to a territorial sea, exclusive economic zone or continental shelf.

Major construction at three of China’s large man-made islands in the Spratly’s has wrapped up, according to a separate AMTI report from March, allowing Beijing to deploy fighter jets and mobile missile launchers to the area at any time.

The building of military and dual-use infrastructure on the three biggest islands in the contested Spratly chain — Mischief, Subi and Fiery Cross reefs — has reached the final stages, with naval, air, radar and defensive facilities largely complete, the report said.

All three islands boast hangers that can accommodate 24 fighter jets and four larger planes, including surveillance, transport, refueling or bomber aircraft. Hardened shelters with retractable roofs for mobile missile launchers have also been built on the three.

China has also constructed significant radar and sensor arrays on all three, positioning them close to point defense structures to provide protection against air or missile strikes.

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