Three Chinese government ships have sailed into the waters of disputed Tokyo-controlled islands, according to Japan's coastguard.
The maritime surveillance ships entered the 12-nautical-mile zone off the Senkaku islands, which China calls the Diaoyu, about 11am (0200 GMT) on Sunday.
Chinese government ships have frequently sailed around the five islands in recent months, causing 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 decision caused an angry response, with Japan's Prime Minister Shinzo Abe pledging to "expel by force" any Chinese landing on the archipelago, which is believed to harbour vast natural resources below its seabed.
The developments come as China's first passenger cruise made its way to the South China Sea Paracel Islands to which several East Asian countries have laid claim.