Tensions between the U.S. and China flared again on Sunday as two U.S. Navy warships cruised near South China Sea islands claimed by Beijing.

The guided-missile destroyer Higgins and the guided-missile cruiser Antietam sailed within 12 nautical miles of the disputed Paracel Islands during a scheduled freedom of navigation operation, Reuters reported.

The move angered Chinese officials, who claim the island group as sovereign territory.

China’s Defense Ministry said it sent ships and aircraft to warn the U.S. vessels to leave the area. The U.S. military did not directly comment on the incident, but maintained its right to conduct routine and regular FONOPS in the region.

During a stop in Hawaii to mark a change in leadership at U.S. Pacific Command, Defense Secretary Jim Mattis said Tuesday that the U.S. will continue to confront China's militarization of manmade islands in the South China Sea.

Mattis said Beijing hasn’t abided by its promise not to put weapons on the Spratly Islands.

He said American ships are maintaining a “steady drumbeat” of naval operations around disputed islands, and “only one country” seems to be bothered by the vessels’ activities.

Mattis said the U.S will confront “what we believe is out of step with international law.”

An MH-60R Seahawk helicopter lands aboard the Ticonderoga-class guided-missile cruiser USS Antietam (CG 54) on May 28, 2014, in the Pacific Ocean. (Mass Communication Specialist Seaman David Flewellyn/Navy)
An MH-60R Seahawk helicopter lands aboard the Ticonderoga-class guided-missile cruiser USS Antietam (CG 54) on May 28, 2014, in the Pacific Ocean. (Mass Communication Specialist Seaman David Flewellyn/Navy)

China has controlled the Paracels entirely since violently seizing Vietnam's holdings in the area in 1974. Called "Xisha" in Chinese, the islands have been incorporated into the southern province of Hainan and are being developed for tourism, as well as being equipped with weapon systems meant to enforce China's claim to virtually the entire South China Sea.

The incident over the weekend comes at a time of mounting tensions between the two nations. The Pentagon officially uninvited China from this year’s Rim of the Pacific naval exercise last week, citing the country’s “continued militarization of disputed features in the South China Sea.”

The Chinese air force announced two months ago that it had landed long-range H-6K bombers on islands in the South China Sea.

That announcement came not long after China deployed advanced fighter jets to the contested region for the first time on public record.

Information from the Associated Press was included in this report.