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Three Chinese government ships sailed into the waters of disputed Tokyo-controlled islands on Sunday, Japan's coastguard said.
The maritime surveillance ships entered the 12-nautical-mile zone off the Senkaku islands, which China calls the Diaoyu, around 11:00 am (0200 GMT).
Chinese government ships have frequently sailed around the five islands in recent months, sparking diplomatic clashes with Japan.
In late April, eight Chinese government vessels sailed into the disputed waters, the biggest flotilla in a single day since Tokyo nationalised part of the island chain in September.
The move sparked an angry response, with Japanese premier Shinzo Abe vowing to "expel by force" any Chinese landing on the archipelago, which is believed to harbour vast natural resources below its seabed.