Vietnam has responded to indications that Beijing is erecting a radar system on an island in the disputed South China Sea.

Doan Khac Viet, deputy spokesperson at the Vietnamese foreign ministry, said on Thursday, "Vietnam is deeply concerned about this information and strongly opposes all actions violating Vietnam's sovereignty over the Paracel Islands."

A report by the London-based Chatham House think tank says China is building radar infrastructure on Triton in the Paracel archipelago. The island is claimed by Vietnam and controlled by China.

The report says the equipment would "significantly increase" China's surveillance capabilities as it's said to be able to detect stealth aircraft.

The two countries recently locked horns near the Paracels. Hanoi lodged a protest with Beijing over what it says was an attack on a Vietnamese fishing boat by Chinese law enforcement in September that left crew injured.