About 100 Chinese-registered boats have been detected encroaching in Malaysia's waters in the disputed South China Sea, Malaysia's state news agency reported on Friday.

The reported encroachment on Thursday is the latest action by Chinese vessels to raise concern in Southeast Asia, where four countries object to China's claim to virtually the whole of the South China Sea.

Malaysia's national security minister Shahidan Kassim said assets from the Malaysian Maritime Enforcement Agency and the navy have been sent to the area near the Luconia Shoals to monitor the situation, the Bernama news agency reported.

Shahidan did not specify what type of Chinese vessels had been spotted.

China claims most of the South China Sea through which about $5 trillion in ship-borne trade passes every year. China's Southeast Asian neighbours Brunei, Malaysia, the Philippines and  Vietnam, also claim parts of the sea, as does Taiwan.

China's foreign ministry spokesman, Hong Lei, asked about the Malaysian report at a regular briefing on Friday, said he did not "understand the details" of what the Malaysian government had said about the matter.

"What I want to point out is that now is the fishing season in the South China Sea ... At this time of year, every year, Chinese trawlers are in the relevant waters carrying out normal fishing activities," Hong said. He did not elaborate.

Shahidan said Malaysia would take legal action if the ships were found to have trespassed into its exclusive economic zone, Bernama cited him as saying.

This week, Indonesia protested to China about an incident involving an Indonesian patrol boat, and a Chinese coastguard vessel and fishing boat in what Indonesia said was its waters.

China has said its vessels were operating in "traditional Chinese fishing grounds" and its coastguard vessel did not enter Indonesian waters.

Indonesia is not embroiled in rival claims with China over the South China Sea and has instead seen itself as an "honest broker" in disputes between China and its neighbours.

China says it is committed to resolving disputes in the South China Sea peacefully, but only via bilateral talks. China has refused to participate in an international arbitration case bought by the Philippines against China.

Speaking at the Boao Forum on China's Hainan island, Chinese Vice Foreign Minister Liu Zhenmin repeated that opposition and rejected the idea of multilateral talks.

"It can be seen from international practice, that apart from after wars, we rarely see multilateral negotiations are able to resolve complex and sensitive territorial and maritime boundary disputes," Liu said, according to a transcript provided by China's foreign ministry.