The Japanese coastguard said three Chinese patrol boats had approached a chain of islands at the centre of a bitter territorial dispute on Wednesday in a move that could inflame tensions.
The vessels entered Japanese waters around the islands in the East China Sea known as Senkaku
in Japanese and Diaoyu in Chinese early morning, the coastguard said.
The trio of Chinese ships were identified as Yuzheng 35001, Yuzheng 204 and Yuzheng 202, it said, adding that "we are demanding that the ships leave our nation's territorial waters".
"The same communication is being made by the foreign ministry, via diplomatic channels," the coastguard added.
The incident comes amid increasingly sore relations over the islands, which lie in rich fishing grounds and may contain valuable mineral reserves, with Tokyo earlier saying it may buy the chain from their private Japanese owner.
The Tokyo metropolitan government said it had already collected more than 1.3 billion yen (USD 16.3 million) in donations from across the country to fund the purchase.
Beijing has shot back that Japan has no right to buy the islands.
Japanese and Chinese foreign ministers were expected to meet later today on the sidelines of an Association of Southeast Asian Nations (ASEAN) forum in Phnom Penh.
The waters around the disputed islands have been scene of previous territorial spats, including the arrest of a Chinese trawlerman in late 2010 when he rammed his boat into two Japanese patrol boats.
The latest chapter in the ongoing spat comes as a separate report in Japan's top-selling Yomiuri Shimbun newspaper implicated a senior member of China's People's Liberation Army in a cyberattack on Japan's Parliament last year.
An email address tied to the unnamed man was allegedly used as a destination for stolen data from the attack which reports had earlier said were launched from a China-based server.
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