President Rodrigo Duterte has said he will "carry the voice of Asean" and raise the issue of the arms build-up on the islands China occupies in the South China Sea with his Chinese counterpart, Mr Xi Jinping.

A Washington-based think-tank recently released a map which showed that the Philippines was within range of Chinese warplanes and missiles being deployed in the South China Sea.

"I am chair of Asean, and I have to carry the voice of Asean. I have to tell the truth that everybody is worried," Mr Duterte told a news briefing on the sidelines of the Asia-Pacific Economic Cooperation meetings in Danang, Vietnam, on Thursday.

He said he would tell Mr Xi at the meeting today that "the whole of Asean is worried about how we should behave in the seas that are now militarised, afraid that there might be a mistake, and there would be shooting".

"Those arms, they are not there for any other purpose. They are not decorations. They are there because China would need them. For what purpose? I really don't know," said Mr Duterte.

China already has Chengdu J-10 multirole fighters, YJ-62 anti-ship cruise missiles and HQ9 surface-to-air missiles stationed on Woody Island in the Paracel chain of islands near Vietnam.

The Asia Maritime Transparency Initiative said Palawan province in the western Philippines and even metropolitan Manila would be within reach once these weapons were in the Spratlys archipelago.

China has laid claim to two-thirds of the South China Sea, a vital sea lane through which US$5 trillion (S$6.8 trillion) in ship-borne trade passes every year.

The Philippines, Vietnam, Malaysia, Brunei and Taiwan have their own claims in the waterway that overlap with China's.

Mr Duterte said Asean and China needed to conclude a "code of conduct" laying down rules on how to settle disputes in the contested waters as quickly as possible.