Japan's Defence Minister Itsunori Onodera (C) arrives to inspect the Ground Self-Defense Force's annual new year military exercise at Narashino exercise field in Funabashi, east of Tokyo, Japan January 12, 2018. REUTERS/Kim Kyung-Hoon

TOKYO (Reuters) - Japan protested to China on Friday after a foreign submarine in waters near disputed islands in the East China Sea was confirmed to be Chinese, calling it a unilateral action and a “serious escalation” that raised bilateral tensions.

The submarine was first detected on Thursday in waters contiguous to Japan’s territory around the islands, known as the Senkaku in Japan and Diaoyu in China, but was finally identified on Friday, the Defence Ministry said, a day after Japan lodged a protest with China over the presence of a frigate in the same area.

Neither entered what Japan considers its territorial waters.

Japan and China have long been at loggerheads over the tiny, uninhabited islands. They are controlled by Japan but claimed also by China.

“The presence of the submarine in Japan’s contiguous waters, along with that of the frigate, has unilaterally raised tensions. We are deeply concerned,” Defence Minister Itsunori Onodera told reporters.

Japanese Vice Foreign Minister Shinzuke Sugiyama telephoned China’s ambassador to Japan, Cheng Yonghua, to lodge a strong protest and demand measures be taken to prevent a repeat, the ministry said in a statement.

China routinely rejects Japanese criticism of such patrols, saying its ships have every right to operate in what China calls its territorial waters.