Six Chinese government ships sailed into waters around disputed islands claimed by both Beijing and Tokyo early Friday, the Japanese coastguard said, adding it had issued warnings telling them to leave.
The arrival came just days after the Japanese government completed its planned nationalisation of the islands, which it administers and knows as Senkaku, but which China claims as Diaoyu.
“Our patrol vessels are currently telling them to leave our country’s territorial waters,” the coastguard said in a statement.
Under international law, territorial waters extend up to 12 nautical miles from the coast of a landmass.
Prime Minister Yoshihiko Noda vowed to maintain utmost vigilance after the coast guard said that two Chinese maritime survey ships entered Japanese waters around 6:18 am (2118 GMT Thursday).
They were followed by a group of four other ships that sailed into the waters claimed by Japan, shortly after 7:00 am, the coastguard said.