The vessel passed through a narrow strip of waters between the prefecture’s Tokara island chain and Amami Oshima without surfacing, according to the ministry.

It was believed to be a Chinese Navy submarine, but the Defense Ministry refused to formally disclose this or its type because that could provide clues about the MSDF’s detection abilities.

Under international law, submarines must surface and raise their national flags inside foreign territorial waters. But underwater cruises are not banned in contiguous zones, which surround territorial waters.