The People’s Liberation Army said it warned a US warship to "leave" waters around the Xisha Islands in the South China Sea on Thursday, saying the craft had illegally entered the country’s territorial waters.

The Southern Theater Command of the PLA said the USS Milius, a guided missile destroyer, had on Thursday entered the area.

The PLA "organised sea and air forces to track and monitor [the ship] in accordance with the law" and "warned it to leave", spokesman Tian Junli said.

The vessel "made an illegal incursion into Chinese territorial waters... without permission from the Chinese government, harming peace and stability" in the region, he said.

"The theater forces will maintain a high state of alert at all times and take all necessary measures to resolutely safeguard national sovereignty and security and peace and stability in the South China Sea," Tian added.

The US military denied the claims, saying the vessel "is conducting routine operations in the South China Sea and was not expelled".

"The United States will continue to fly, sail, and operate wherever international law allows," said a spokesperson for US Indo-Pacific Command. (AFP/Reuters)