Japan said Monday that a Chinese naval submarine spotted in waters off flashpoint islands in the East China Sea was one of its new types of nuclear-powered attack vessels.
Tokyo's statement comes on the same day that China announced three of its "Coast Guard vessels conducted a patrol in territorial waters off the Diaoyu Islands," Beijing's name for the contested isles called Senkaku in Japan.
Japan launched an official protest last Thursday after their navy spotted the 4,000-tonne Jiangkai II class frigate and an unidentified submarine in waters surrounding the Tokyo-administered islands.
Japanese Defence Minister Itsunori Onodera told reporters Monday the submarine has been determined to be "China's Shang-Class nuclear-powered attack submarine," which he said can be equipped with long-range cruise missiles.
"Nuclear-powered submarines can also cruise for long hours and it is more difficult to detect them because they dive deep," Onodera said.
"We have serious concerns as the submarine's underwater passing through our country's contiguous waters is an act that unilaterally increases tension," he added, noting that Japan would stay vigilant.
Contiguous waters are a 12-nautical-mile band that extends beyond territorial waters.
China has not confirmed that it had sent a submarine.
In Beijing, foreign ministry spokesman Lu Kang told a regular press briefing last week "the Chinese naval vessels conducted surveillance over the activities of the Japanese side" and repeated China's claim to the islands.
"As for the submarine, I'm not aware of the relevant issue," he added.
Relations between Japan and China deteriorated in 2012 when Tokyo "nationalised" some of the islets.
Since then, the two top Asian economies have taken gradual steps to mend fences but relations remain tense.
Chinese coastguard vessels routinely travel around the disputed islands.
The incident came as Japan is pushing to host a trilateral summit with leaders from China and South Korea.