The Defense Ministry said Saturday that a foreign submarine headed west underwater in a contiguous zone northeast of Amami Oshima Island, in Kagoshima Prefecture, on Thursday afternoon.
Maritime Self-Defense Force destroyers and patrol aircraft monitored the submarine, which was traveling west outside a contiguous zone west of the prefecture's Yokoate Island on Saturday morning, according to the ministry.
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.
The contiguous zone is a band of water beyond the 12 nautical mile territorial waters surrounding any national territory and extends out to 24 nautical miles.
Defense Minister Taro Kono issued instructions to make every possible effort to collect information and conduct vigilance and surveillance.
The activities involved MSDF patrol aircraft of the Kanoya base in Kagoshima and the Naha base in Okinawa Prefecture, as well as the Ashigara Aegis ship of the Sasebo base in Nagasaki Prefecture and the Kaga helicopter carrier from the Kure base in Hiroshima Prefecture.
The submarine "may have tested Japanese and U.S. anti-submarine warfare capabilities," an SDF source said.
In January 2018, Tokyo announced it had detected a Chinese nuclear-powered attack submarine near the Japan-administered Senkaku Islands. The uninhabited islands in the East China Sea are also claimed by Beijing, which calls them Diaoyu, and Taiwan, which calls them Tiaoyutai.
The event — the first time a Chinese sub had been observed in the Senkakus' contiguous zone — triggered alarm in Tokyo, which then complained to Beijing.