InsideJapan News Network

Japan's defence ministry has said it is mystified about the appearance of an unidentified submarine off the country's coast.

The periscope of the mystery sub is believed to have been spotted by crew members of the Japanese destroyed Atago, reports Japan Today.

Atago's sonar was able to confirm that the sub did not belong to Japanese navy or the allied US navy.

It is not the first time such an incident has occurred. A Chinese nuclear submarine entered Japanese territorial waters near the Miyako island chain in 2004.

When asked why the sub was allowed to intrude so far into Japanese waters, a self-defence force official commented: "That's one of the points we have to address hereafter. We can't provide any answers at the moment."

Defence minister Yoshimasa Hayashi said he would make an official protest to the relevant country once the nationality of the vessel is identified.