The People's Liberation Army's Southern Theater Command warned off a United States warship that illegally entered China's territorial waters in the South China Sea for the second time in two days, the Defense Ministry said on Friday.

"The USS Milius on Friday entered China's territorial waters near the Xisha Islands again without the Chinese government's authorization, undermining peace and stability in the region," Senior Colonel Tan Kefei, spokesman for the Ministry of National Defense, said in a statement.

The Southern Theater Command monitored the unlawful entry of the US warship into China's territorial waters and warned it off, he said.

According to a prior news release by the theater command, the US warship also illegally entered the area on Thursday. The ship was monitored and warned off by the Chinese military.

Tan said the warship's actions gravely infringed on China's sovereignty and security and violated international law.

"It is more ironclad proof of the US's hegemonic navigation and militarization of the South China Sea. We solemnly demand that the US immediately stop such provocative actions," he said, warning that, otherwise, the US would bear the serious consequences of accidents caused by such acts.

The PLA will take all necessary measures to firmly safeguard national sovereignty and security as well as peace and stability in the South China Sea, the spokesman stressed.