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.