Philippine President Rodrigo Duterte has said his nation could go to war if its soldiers were hurt in the disputed South China Sea, a top aide said Wednesday after allegations emerged that Beijing had harassed Manila's troops in the area.

China has voiced "strong dissatisfaction" after two US warships sailed by an island claimed by Beijing in the disputed South China Sea, adding to simmering tensions in the strategic waterway.

Vietnam and China agreed Monday to avoid conflicts in the hotly contested South China Sea, as a new pathway to dialogue on easing tensions was opened with other Southeast Asian nations.

An American warship on Sunday sailed close to a disputed island in the South China Sea occupied by Beijing, a US official said, a provocative move that could further strain relations between the superpowers.

China has expressed "firm opposition" to remarks made by US Pentagon chief Jim Mattis during a regional defence summit over the weekend, after he criticised Beijing's "militarisation" of the South China Sea.

China on Thursday accused the US of trespassing after an American warship sailed near a reef claimed by Beijing in the South China Sea, the first such operation by President Donald Trump's administration in the disputed waterway.

Philippine President Rodrigo Duterte said Friday China's leaders told him they were prepared to go to war over competing claims in the South China Sea.

Philippine President Rodrigo Duterte said Thursday he has ordered troops to deploy on unoccupied South China Sea islands, boosting the military presence on remote reefs claimed by Manila in a move that could provoke rival claimants including Beijing.

China Wednesday denied plans to build an environmental monitoring station on a disputed shoal near the Philippines' coastline, after a local official last week announced the plan.

The White House warned China it will "defend" US and international "interests" in the South China Sea and that trade must be a "two-way street."

China offered a muted response Thursday after Donald Trump's secretary of state pick Rex Tillerson warned the US would stop it from using its artificial islands in the South China Sea.

China said Thursday that the deployment of weapons in the South China Sea had "nothing to do with militarisation", calling the construction of defensive facilities "normal".