Unidentified Submarine In Japanese Territorial Waters

The Japanese Maritime Self-Defense Forces reported that one of their destroyers spotted the periscope of an "unidentified" submarine in their home waters between Kyushu and Shikoku islands yesterday:

Japan's navy tracked a submarine that intruded into its waters for nearly two hours Sunday and then lost track of it without identifying its nationality, a military official said.
Naval vessel Atago spotted the submarine in Japanese waters at 6:56 am (2156 GMT Saturday) but the craft did not raise a national flag or surface, breaching international laws, a defence ministry spokesman said.
Atago, an Aegis-equipped warship, chased the submarine off the coast of Kochi prefecture in western Japan, facing the Pacific Ocean, until about 8:40 am (2340 GMT), he said.
"We have not identified the nationality of the submarine and we are still searching for it," the spokesman said.
Using sonar, Atago confirmed that the submarine does not belong to the Japanese navy nor its ally the US navy, he said.

We last heard about JDS Atago (DDG 177) earlier this year when they bumped a fishing vessel. While the unidentified submarine could be Russian or Chinese, past history would indicate that China is the most likely violator. The main evidence for that is fairly obvious -- if the Japanese destroyer was able to "confirm" that the sub wasn't from the U.S. with sonar, that means if must have been loud enough to be heard by a skimmer's sonar, and that's a distinguishing characteristic of Chinese SSNs.