It was the first time China vessels entered the waters since Dec 18: Japan Coast Guard (Representational)
The Chinese coastguard said it patrolled the territorial waters of the Diaoyu Islands on Thursday to conduct "right protection".
The East China Sea islets are claimed by both China and Japan, which calls them the Senkaku Islands and have long been a sticking point in bilateral ties.
Japan's Coast Guard separately said in a bulletin that it had repeatedly urged the four Chinese coastguard vessels to leave "our territorial waters".
The four Chinese ships left the waters after almost two hours and sailed south, Japan's Coast Guard said in a later bulletin.
It was the first time Chinese coastguard vessels had entered the waters since Dec. 18, Japan's Coast Guard said.
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