This post has been updated to indicate that the freedom of navigation operation past Mischief Reef was the second, not the first FON op this year. A January mission past Scarborough Shoal was considered a freedom of navigation operation.

A U.S. guided-missile destroyer conducted a freedom of navigation operation past an artificial island controlled by China on Friday, a U.S. defense official confirmed to USNI News.

USS Mustin (DDG-89) came within 12 nautical miles of Mischief Reef, an artificial island in the Spratly Island chain that is home to a major airfield, and conducted maneuvers near the Chinese base. Newswire Reuters first reported the operation on Friday.

“China holds indisputable sovereignty over the islands and their surrounding waters in the South China Sea,” defense ministry spokesman Ren Guoqiang said in a statement via CNN.

“By repeatedly sending military ships into these areas without authorization, the U.S. has seriously harmed Chinese sovereignty and security, violated basic rules of international relations, and harmed regional peace and stability.”

China claimed two People’s Liberation Army Navy frigates warded off the U.S. destroyer.

In a statement that did not acknowledge details of the operation, the U.S. Pacific Fleet said it conducts operations within the bounds of international law.

“All operations are designed in accordance with international law and demonstrate that the United States will fly, sail and operate wherever international law allows. We conduct routine and regular Freedom of Navigation Operations (FONOPs), as we have done in the past and will continue to do in the future,” read a statement from Lt. Cmdr. Nicole Schwegman provided to USNI News.

“FONOPs are not about any one country, nor are they about making political statements. The United States takes a strong position on protecting the rights, freedoms and lawful uses of the sea and airspace guaranteed to all countries and that all maritime claims must comply with international law as reflected in the Law of the Sea Convention.”

Of the artificial islands Beijing has built in the South China Sea, an operation past Mischief Reef sends the least ambiguous challenge to China’s claims in the region. The installation is built on a low tide elevation – a feature that is underwater at high tide – and under the U.N. Law of the Sea Convention doesn’t command a territorial sea. Though not specified by the official, previous missions past Mischief Reef have specifically challenged China claim of a territorial sea for the feature.

The last known challenge to Mischief occurred in August by guided-missile destroyer USS John S. McCain (DDG-56). In May, USS Dewey (DDG-105) came within six nautical miles of the base on Mischief Reef and spent 90 minutes within 12 nautical miles of the base.

USNI News understands this freedom of navigation mission is the second known FON op for the U.S. Navy in 2018. A January presence mission in which USS Hopper (DDG-70) operated near the Chinese-claimed Scarborough Shoal off the Philippines was part of the freedom of navigation program, a defense official confirmed to USNI News on Friday.