Japan was searching Sunday for an unidentified foreign submarine detected in its territorial waters earlier in the day, the Defense Ministry said.
The Aegis destroyer Atago of Japan's Maritime Self-Defense Force spotted what it determined to be a periscope of an unknown submarine between Kyushu and Shikoku islands in western Japan, the ministry said.
By the time officials confirmed that the submarine was not a U.S. or Japanese vessel, it had left the area, it said.
Officials dispatched the Atago as well as P-3C patrol airplanes to look for the submarine, the ministry said in a statement.
"We need to do our utmost to track down the submarine and get to the bottom of the incident," Defense Minister Yoshimasa Hayashi was quoted by Kyodo News agency as saying.
Under the United Nations Convention on the Law of the Sea, foreign submarines and other underwater vehicles are "required to navigate on the surface and to show their flag" in territorial waters during peacetime.
In November 2004, a Chinese nuclear-powered submarine entered Japanese territorial waters near the southern island of Okinawa and ignored Japanese orders for it to surface. The incident escalated already stormy relations with China at the time, stirring up fear in Japan over China's military.
That submarine's presence prompted the Maritime Self-Defense Force to go on alert for the second time since World War II.
Hayashi said Sunday's incident was not serious enough to call for similar security operations, according to Kyodo.