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".
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