Japan's navy went on alert Wednesday when an unidentified submarine made a brief incursion into the country's southern waters near Okinawa.
The submarine left Japanese waters shortly after it was spotted, and a reconnaissance aircraft and destroyer were monitoring its movements, Chief Cabinet Secretary Hiroyuki Hosoda said. Tokyo also was trying to determine where the vessel came from.
If the submarine's origin is identified, Japan will take "necessary steps," Hosoda said, without elaborating.
Japan's public broadcaster NHK said defense officials were investigating a possible link between the sub sighting and Chinese military vessels detected recently in Japan's southern waters.