Japan lodged a protest against Beijing on Friday after four armed Chinese coast guard vessels entered waters that Tokyo considers its territory.

The Japanese government said it was the first time that four Chinese vessels carrying what appeared to be cannons had entered its territorial waters in the East China Sea surrounding the disputed islands, which Tokyo calls the Senkakus and Beijing calls the Diaoyu.

"I am not in a position to state what the Chinese side's intentions are, but the intrusion of vessels belonging to China's coast guard into (our) territory is a breach of international law," Chief Cabinet Secretary Yoshimasa Hayashi told a news conference.