The People's Liberation Army monitored a United States warship that illegally entered the waters near China's Ren'ai Reef on Monday, a PLA spokesperson said.

Tian Junli, spokesperson for the PLA's Southern Theater Command, said in a statement on Monday morning that the littoral combat ship USS Gabrielle Giffords illegally entered waters near Ren'ai Reef in China's Nansha Islands without the approval of the Chinese government.

He said that the theater command organized maritime forces to monitor the warship's passage.

Tian said the US deliberately stirred up trouble in the South China Sea, seriously violated China's sovereignty and security, seriously undermined regional peace and stability, and seriously violated international law and basic norms governing international relations, fully demonstrating that the country is the biggest threat to peace and stability in the region.

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

He warned that the troops of the theater command will remain on high alert at all times and resolutely safeguard national sovereignty and security and maintain peace and stability in the South China Sea.