Japan today detected a submarine of unknown origin in its territorial waters, the Government's top spokesman said.

Surveillance planes spotted the submarine in waters off a southern island near Okinawa, Chief Cabinet Secretary Hiroyuki Hosoda told a news conference.

He said Japan was trying to find out more about the vessel's origin.

ANALYSIS: As it's still being tracked, it's very likely to be a diesel submarine, even a primitive nuke would probably have evaded by now. It may not be North Korean, but given previous incidents, it's likely.
It is an open secret that submarines of all nations routinely violate territorial waters when performing surveillance and covert ops missions.

A Japanese navy aircraft chased an unidentified submarine out of waters near Japan's Okinawa island chain on Wednesday in a rare display of the country's military capability, long constrained by a pacifist constitution.

The P3C patrol plane that spotted the sub near the islands 1,000 miles southwest of Tokyo was still tracking the vessel, which had been ordered to surface and show a flag even though it had left Japanese waters, the government said.

Several Japanese destroyers had also been dispatched to the area, Kyodo news agency reported.

"It is regrettable. It is certainly not a good thing that an unidentified submarine entered our country's territorial waters," Prime Minister Junichiro Koizumi told reporters.

Some media reports suggested the submarine may be Chinese.

Public broadcaster NHK quoted an official at the Chinese embassy in Tokyo as saying he did not know whether Chinese military vessels had entered Japanese waters.

It is rare for the Maritime Self-Defense Force, as the navy is known, to be mobilized over such incidents.

The Japan Coast Guard is in charge of maintaining the security of the country's coastline and waters.

The last time the navy was mobilized was in 1999 when a suspected North Korean spy ship violated Japanese waters.

In December 2001, an unidentified ship also believed to have been crewed by North Korean spies sank in the East China Sea after an exchange of fire with a coast guard patrol vessel.