Japan summoned the Chinese ambassador in Tokyo early on Thursday to express concern after a Chinese navy ship sailed close to what Japan considers its territorial waters in the East China Sea for the first time, the Foreign Ministry said.
The Foreign Ministry said the Chinese navy ship had entered waters shortly after midnight that are contiguous to what Tokyo considers its territory in the vicinity of disputed islands, known as the Senkaku in Japan and the Diaoyu in China.
The incident came as a potential setback after signs of improvements seen in relations between Asia's two largest economic powers, which had soured in recent years over their wartime history and territorial rows.
Japan's Vice Foreign Minister Akitaka Saiki summoned the Chinese ambassador in Tokyo at around 2 a.m. (1.00 p.m. ET on Wednesday) to "express a serious concern" and urge the Chinese ship to leave the area, the ministry said in a media statement.
The Chinese frigate left the waters about an hour later, sailing to the north, the Japanese Defence Ministry said.
Chinese patrol ships have occasionally sailed close to or entered what Tokyo considers its territorial waters, but this was the first time that a Chinese naval ship made its way into those waters.
At around the same time, three Russian battleships entered waters close to what Japan considers its territory, the Defence Ministry said. The Ministry said it was investigating whether the Chinese and Russian actions were related.
None of the ships violated what Japan considers its territorial waters, it said.
The uninhabited East China Sea islands lie about 220 km (135 miles) west of Taiwan and are controlled by Tokyo but claimed by Beijing.
There was no immediate comment from Beijing but China in the past has warned Japan not to take any "provocative" action over the disputed islands. Chinese ships sailed near the disputed islands last year.
Japan in turn in the past has told China that any foreign naval vessel that entered Japanese waters for reasons other than "innocent passage" would be told to leave by a Japanese naval patrol.
