Japan will "strengthen security" around its coasts, Prime Minister Yoshihiko Noda said on Monday, as Chinese ships again plied waters near disputed islands.

"While observing the pacifism that is a pillar of our constitution... I will make efforts in strengthening security in surrounding sea areas," Noda said in a policy speech to parliament.

"It is unmistakable that the security environment surrounding Japan is becoming more serious than ever. Various events touching on territorial and sovereign rights are occurring," he said.

The comments came after Japan said on Friday that it will spend 17 billion yen ($213 million) to beef up its coastguard, as maritime confrontations continue over an archipelago in the East China Sea.

"With an unflagging resolve, I will carry out the function of protecting territorial lands and waters... while observing international law," said Noda.

On Sunday, four Chinese maritime surveillance ships were spotted in territorial waters around the Tokyo-controlled Senkaku islands, which China claims as the Diaoyus.

On Monday Chinese vessels were in the so-called contiguous zone, a band of waters that stretches 12 nautical miles from the edge of a state's territorial waters.

Chinese vessels have moved in and out of what Japan says is its sovereign territory over the last nearly two months since Tokyo nationalised some of the islands.

As well as the potential mineral reserves to which ownership of the islands grants access, both countries have considerable amounts of national pride at stake in the decades-old spat.

The dispute has hit the huge trade relationship between the two largest economies in the region and senior representatives from both governments are reportedly readying for a third round of talks on the issue.