A foreign submarine was detected on May 12 plying the depths in a contiguous zone outside Japan's territorial waters south of Kumejima island in Okinawa Prefecture, the Defense Ministry said May 13.
Although officials did not elaborate on the sub's country of origin, a government source said the vessel likely belonged to the Chinese Navy.
"I was prepared to order 'maritime security operations' immediately upon getting approval from the prime minister (Shinzo Abe), if the submarine entered (Japan's) territorial waters," Defense Minister Itsunori Onodera told reporters May 13.
Under the Self-Defense Forces Law, the defense minister has the authority to order, subject to the prime minister's approval, SDF units to conduct "necessary operations" to protect lives and property at risk and to maintain security at sea.
Such circumstances, called "maritime security operations," allow the SDF to use weapons in lawful self-defense and emergency evacuation.
The last time maritime security operations were ordered against a submarine was in November 2004, when a submerged Chinese sub entered Japan's territorial waters around the Sakishima island chain in Okinawa Prefecture. Although the latest case did not involve a submarine incursion into territorial seas, Onodera is believed to have mentioned the measures to warn any countries involved.
International law does not prohibit submarines from entering a contiguous zone. But the ministry officials said they decided to make the latest incident public because they concluded it was an unusual case, which involved passage through the contiguous zone over a prolonged period.