China has deployed four patrol ships to a disputed area of the South China Sea, state media said Sunday, amid a deepening row with Vietnam over competing territorial claims.

The ships, described by the Xinhua news agency as surveillance vessels, reached what China calls the Huayang reef in the Spratly islands on Sunday.

China last month summoned Vietnam's ambassador in Beijing and protested a law adopted by the Vietnamese parliament that places the disputed Spratly islands under Hanoi's sovereignty.

China and Vietnam, as well as other neighbouring nations, are locked in long-standing territorial disputes over the South China Sea, including the resources-rich Spratly and Paracel islands.

Xinhua said that the ships left China's southern island province of Hainan on June 26 and would travel more than 2,400 nautical miles on patrols.

The ships are under the authority of the Chinese government's State Oceanic Administration and not the country's navy.

China said Thursday it would resolutely oppose any military provocation in its territorial waters and protect its sovereignty -- remarks which appeared to be directed partly at Vietnam.

China's military has established routine naval patrols in the South China Sea as a matter of "national sovereignty", defence ministry spokesman Geng Yansheng said.

"The determination and will of China's military to safeguard national sovereignty and territorial integrity is unwavering," he said.

China said last month it had elevated the administrative status of what it calls the Nansha (Spratly) and Xisha (Paracel) islands from a county to a prefectural-level district.

China's state-backed China National Offshore Oil Corp. has also called for tenders from foreign companies to explore for oil in the South China Sea near Vietnam, a move Hanoi deemed "illegal".

Tensions in the South China Sea have risen sharply recently, with China and the Philippines also locked in a maritime dispute over Scarborough Shoal, a reef off the Philippine coast.

Taiwan, Vietnam, Brunei, China, Malaysia, and the Philippines all claim parts of the South China Sea.