Tokyo: Chinese coast guard vessels sailed into territorial waters around disputed islands in the East China Sea on Saturday, Japan's coast guard said, marking the second such incident in a week.
The four ships entered the waters surrounding the island chain, controlled by Japan and known as Senkaku in Japanese and Diaoyu in China, at around 10:30 am (0130 GMT) and left within two hours, according to the coast guard.
A similar incident last Sunday prompted Tokyo to lodge a complaint with China's foreign ministry through its embassy in Beijing, saying the uninhabited islets are "an inherent territory of Japan".
Japan has made dozens of protests through diplomatic channels this year over what it says have been more than 30 days of intrusions by Chinese vessels in the contested waters.
The two countries have been locked in a long-running dispute over the islands, which are believed to harbour vast natural resources below their seabed, with China claiming them as its own.
Beijing is also involved in maritime disputes in the South China Sea.
It asserts sovereignty over most of the area despite rival partial claims by the Philippines, Brunei, Malaysia, Taiwan and Vietnam.
A UN-backed tribunal in July ruled that Beijing's claims were invalid.