Three Chinese government ships entered waters around the disputed Diaoyu Islands in the East China Sea yesterday, the Japanese coastguard said.
The Chinese surveillance vessels approached within 12 nautical miles of the island of Uotsurijima in the island chain - known as the Senkakus in Japan - shortly before 8.30am and left three hours later, the coastguard said.
There have been several similar incidents in recent months.
Yesterday's took place after former Japanese prime minister Yukio Hatoyama came under fire in Japan this week for saying he understood China's claim to the islands.
Hatoyama told Hong Kong-based Phoenix Television on Tuesday it was "unavoidable" that China believed Japan "stole" the islands.
The remark raised eyebrows in Japan. On Wednesday, Japan's top government spokesman, Yoshihide Suga, declared himself dumbfounded.
Goshi Hosono, secretary general of the centre-left Democratic Party of Japan, which Hatoyama helped found, said the remark was "extremely inappropriate" and urged him to reflect on what he had said.
However, Hatoyama reiterated the conciliatory stance in Beijing yesterday and said both sides should set aside the dispute.
"The pressing issue is to encourage the Abe administration to amend its attitude towards other Asian countries, particularly on its view of the history, and to co-operate with other neighbours," he said in a speech delivered at the World Peace Forum. "Referring to the East China Sea Peace Initiative proposed by Taiwanese President Ma Ying-jeou, Japan, China and Taiwan should put their territorial disputes aside and instead focus on issues such as fisheries and environmental protection."
It has long been Beijing's stance that both sides should acknowledge the dispute and set it aside until later.
Hatoyama criticised the decision by a large number of Japanese government politicians to worship at Tokyo's controversial Yasukuni Shrine, which honours Japan's 2.5 million war dead, including 14 class A war criminals from the second world war.
He also attacked Prime Minister Shinzo Abe's refusal to acknowledge Japan's past invasions of Asian countries.
"As a result, not just China and South Korea, but also the US are jointly concerned over his administration's handling of its relationship with other Asian countries," Hatoyama said.
In a recent defence white paper, the Japanese government urged Asian countries and the international community to step up their guard against China's growing military muscle