The USS Milius, an Arleigh Burke-class guided-missile destroyer, "engaged in 'normal operations' within 12 nautical miles of Mischief Reef," the Navy's Seventh Fleet said in a statement, after which the vessel "exited the excessive claim area and continued operations in the South China Sea."

The maritime feature is among three the U.S. believes have been fully militarized by China in the past decade, alongside Subi Reef and Fiery Cross Reef, all in the Spratly Island group. Recent aerial images have revealed extensive infrastructure including runways, hangars, and radars as well as anti-air and anti-ship capabilities.

Beijing has used its claim to every feature in the energy-rich sea to draw straight baselines around archipelagos like the Paracel and Spratly Islands in an attempt to restrict navigation by foreign vessels. It has hindered the lawful commercial activity of other less powerful claimants in the area, in particular the Philippines and Vietnam, with its navy or coast guard. Other nominal claimants of the Spratly Islands include Brunei, Malaysia, the Philippines, Taiwan, and Vietnam. Taiwan and Vietnam also claim the Paracel Islands.

An airfield, buildings, and structures are seen on the artificial island built by China in Mischief Reef on October 25, 2022, in the Spratly Islands of the South China Sea. The United States military sailed a warship near a fortified artificial island in the South China Sea on Monday in order to challenge the Chinese government's unlawful claim to all its surrounding waters, according to the U.S. Navy.

Regional capitals, backed by Washington, argue the practice is inconsistent with the 1982 U.N. Convention on the Law of the Sea, which China has signed and ratified. Beijing has consistently rejected an international arbitral ruling in 2016 that found its maritime claims to be unlawful.

The April 10 freedom of navigation exercise, or FONOP, in the Spratlys "demonstrated that Mischief Reef, a low-tide elevation in it its natural state, is not entitled to a territorial sea under international law," said the Seventh Fleet. "The land reclamation efforts, installations, and structures built on Mischief Reef do not change this characterization under international law."

"Unlawful and sweeping maritime claims in the South China Sea pose a serious threat to the freedom of the seas, including the freedoms of navigation and overflight, free trade and unimpeded commerce, and freedom of economic opportunity for South China Sea littoral nations," it said.

Tian Junli, a spokesperson for the Southern Theater Command of China's People's Liberation Army, said Chinese forces tracked the Milius after it "illegally intruded" into the waters near Mischief Reef "without the approval of the Chinese government."

"China has indisputable sovereignty over the South China Sea islands and their surrounding waters," Tian said.

The Arleigh Burke-class guided-missile destroyer USS Milius conducts routine underway operations on April 10, 2023, in the South China Sea. The Milius conducted a freedom of navigation operation near Mischief Reef, one of China’s three heavily fortified artificial islands.