Japan lodged its second complaint this week with Beijing on Thursday after a Chinese patrol ship neared disputed islands in East China Sea, a day after three other Chinese vessels were seen in the area.

Japanese coastguard said the "Yuzheng 33001" approached the islands, known as Senkaku in Japanese and Diaoyu in Chinese, early Thursday and remained in the area, insisting it was "patrolling Chinese waters".

Tokyo complained over the incursion through diplomatic channels, Chief Cabinet Secretary Osamu Fujimura told reporters.

The incident came the day after three other Chinese vessels had entered the resource-rich waters that Tokyo and Beijing both claim.

That prompted Tokyo to summon the Chinese ambassador and a complaint from Japanese Foreign Minister Koichiro Gemba to his Chinese counterpart during talks in Cambodia.

Chinese Foreign Minister Yang Jiechi responded by reiterating China's claim to the islands.

Fujimura said Thursday that the Japanese coastguard had been ordered to "remain on high alert and continue to monitor the situation".

Despite the importance of the two nations' trading ties, relations between Japan and China are frequently tense, particularly over the territorial dispute and Japan's war-time aggression in Asia.

The uninhabited outcrops were the scene of a particularly nasty spat in late 2010 when Japan arrested a Chinese trawlerman who had rammed two of its coastguard vessels.

A weeks-long standoff in which ties were frozen and Chinese exports of materials vital to high-tech manufacturing were squeezed, was only resolved when Japan backed down and released the fisherman.

Chinese claims to waters far from its coast have increased in frequency and vigour over the last few years as the country has become wealthier and more self-confident.

US Secretary of State Hillary Clinton, who is on a regional tour, said earlier Thursday that nations should settle their territorial disputes "without coercion, without intimidation, without threats, and without use of force".