The Philippines says it has asked China to clarify its reported plans to build an environmental monitoring station on a disputed shoal.

Acting Philippine Foreign Secretary Enrique Manalo told Filipino reporters on Wednesday in Thailand, where President Rodrigo Duterte is visiting, that his department has asked for clarification on the reported planned construction on Scarborough Shoal.

The shoal, off the northwestern Philippines, is at the heart of the territorial disputes between the countries.

Manalo says Duterte has been clear the Philippines wants a peaceful settlement of the disputes, but will act to protect its national interests if necessary.

The top official in Sansha City, which administers China's claims in the South China Sea, was quoted by the Hainan Daily newspaper as saying preparations were underway to build the station.