Japan lodged a formal complaint with China over an incident in the early hours Thursday in which it said a Chinese naval vessel sailed close to disputed islands in the East China Sea.

Reuters cites a statement in which Japanese Foreign Minister Akitaka Saiki “express[ed] a serious concern” to the Chinese ambassador in Tokyo after the warship entered waters close to the Senkaku islands, which are controlled by Japan.

China, which calls them the Diaoyu, claims that the islands — and the surrounding waters — are its historic territory. Relations between the two countries have soured over the dispute since the Japanese government agreed to buy the islands from their previous private owners, a Japanese family, in 2012.

The Chinese vessel reportedly left the waters after an hour, according to the Japanese Ministry of Defense. Reuters says the ministry believes three Russian battleships also came close to Japanese territory at around the same time.

[Reuters]