HONG KONG — An American warship sailed on Tuesday within 12 miles of an artificial island built by China in the South China Sea, an operation intended to show that the United States opposes China’s efforts to restrict navigation in the strategic waterway, the Pentagon said.

The warship, the William P. Lawrence, a guided missile destroyer, ventured into the vicinity of Fiery Cross Reef, a 700-acre artificial island China constructed in the last 18 months on top of two small rocks.

The operation on Tuesday, known as a freedom-of-navigation patrol, came as tensions between the United States and China escalated ahead of a United Nations arbitration ruling on whether Beijing has the right to claim 12-mile territorial waters and 200-mile exclusive economic zones around reefs and atolls in the South China Sea.

The ruling, in a case brought by the Philippines, an American ally, is expected in the coming weeks.

China has built a military-capable runway and dredged a deepwater port on Fiery Cross Reef, one of seven specks in the Spratly archipelago close to the Philippines that it has enlarged. China contends that other countries must request transit rights for their ships around its claims in the South China Sea, the Pentagon said.