Japan's navy went on alert yesterday when an unidentified submarine made a brief incursion into waters near Okinawa, and Tokyo was trying to determine where the vessel came from.
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.
"It's regrettable," Prime Minister Junichiro Koizumi told reporters. "It's certainly not a good thing that a submarine of unknown national origin enters our territorial waters."
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 waters.
Kyodo News quoted unidentified defense officials as saying that the vessel may be a Chinese nuclear submarine.
A P3C reconnaissance plane confirmed that the submarine had entered Japanese territorial waters near the Sakishima islands in Okinawa prefecture, Hosoda said.
After exiting Japanese territory, the aircraft followed the submarine as it cruised international waters in an indeterminate route that did not appear to be straight, Hosoda said.
Defense chief Yoshinori Ono "issued a maritime alert order" and an emergency task force was set up at the prime minister's office, Hosoda said.
Defense officials confirmed that two Chinese military vessels -- a submarine rescue vessel and a towing vessel -- were spotted between Friday and Monday in waters 315km off Tanegashima island. That location is about 1,000km northeast of the spot where the submarine was found yesterday morning.
Officials, however, refused to comment on a possible link between the two incidents.
Japan has been considering ways to boost its maritime defenses after a shoot out with a suspected North Korean spy ship in December 2001.
Top ruling party official Shinzo Abe called the infiltration a "very serious matter" and urged the military to use its abilities to determine where the submarine was from.
The Sakishima islands lie in waters between the northeastern tip of Taiwan and Okinawa's main island.