The Philippine foreign ministry has accused China of reclaiming land on a disputed reef in the South China Sea and said it appeared to be building an airstrip.

Foreign ministry spokesman Charles Jose told Reuters news agency on Wdnesday that China had been moving earth and materials to Johnson Reef in the Spratly Islands over recent weeks.

It also said China was reclaiming land in violation of the Declaration on the Conduct of Parties in the South China Sea, an informal code of conduct for the region.

The ministry had already lodged a protest with the Chinese and raised the issue behind closed doors at last weekend's summit of the Association of South East Asian Nations in Myanmar, Jose said.

A senior Philippine government official, who spoke to AP news agency on condition of anonymity because he was not authorised to talk about the issue, said China could be building an offshore military and resupply and refuelling hub.

The government estimates that the reclamation has turned the submerged reef and a sand bar into a 30-hectare land mass that transformed the underwater outcrop into an islet, a senior Philippino diplomat told the AP on condition of anonymity because of a lack of authority to discuss the issue.

It is the latest territorial spat between the Asian neighbours that have ratcheted tensions in the potentially oil and gas-rich region, which also straddles one of the world's busiest sea lanes.

Vietnam and China have separately been engaged in a dangerous standoff off the Paracels Island after Beijing deployed a mobile oil rig backed by dozens of security vessels.