"On December 4, littoral combat ship USS Gabrielle Giffords illegally entered the waters adjacent to Ren'ai Reef in the Nansha region of China without the approval of the Chinese government," Southern Theatre Command spokesman Tian Junli said.

The Second Thomas Shoal -- referred to in Chinese as Ren'ai Reef -- is about 200 kilometres (125 miles) from the western Philippine island of Palawan, and more than 1,000 kilometres from China's nearest major landmass, Hainan island.

China's military on Monday "followed the entire operation", said Tian, adding that the "deliberate stirring up of the South China Sea by the United States is a serious infringement of China's sovereignty and security".

"Troops in the theatre of command maintain a high state of alert at all times, resolutely defending national sovereignty and security".

Beijing has ignored a 2016 ruling by the Permanent Court of Arbitration in The Hague that its claims to almost the entire sea have no legal basis.

China has ramped up patrols of the waters and reefs in the South China Sea over the past decade or so, and built artificial islands that it has militarised to reinforce its assertion.