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, Reuters reports. 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. It was finally identified on Friday, according to the Defense Ministry. The previous day, Japan lodged a protest with China over the presence of a frigate in the same area. “The presence of the submarine in Japan’s contiguous waters, along with that of the frigate, has unilaterally raised tensions. We are deeply concerned,” Defense Minister Itsunori Onodera said.