Japan filed a formal protest with Beijing Monday, after two Chinese patrol boats were spotted near a disputed island chain in the East China Sea.
Japanese Chief Cabinet Secretary Yoshito Sengoku said Monday that the coast guard spotted the two Chinese boats leaving the disputed waters Sunday night and sailing north toward China.
Beijing and Tokyo have been locked in their worst dispute in years, which started after Japan arrested a Chinese fishing trawler captain on September 8, near the uninhabited island chain.
Japan administers the islands and calls them Senkaku, while China refers to them as Diaoyu. The islands are surrounded by rich fishing grounds and waters thought to hold undersea natural gas and oil reserves.