The submarine was detected in Japanese waters near islands disputed with China about 300 kilometers (180 miles) southwest of Okinawa, officials said. The southern island of Okinawa is home to a major US military base.

Japan was following the submarine with a PC-3 surveillance airplane, a destroyer and at least one navy helicopter, a military spokesman said.

"The submarine is (now) cruising in international waters and it is not necessarily cruising straight. The PC-3 is continuing to follow it," Chief Cabinet Secretary Hiroyuki Hosoda, the government spokesman, told reporters.

The Kyodo News agency, quoting unnamed defense sources, said the vessel was a nuclear submarine from China. Jiji Press said the vessel was in Japanese waters for about three hours.

The incident comes amid a series of disputes between Japan and China, including friction over the right to explore for gas near their maritime border in the East China Sea.

Japanese officials declined to blame China.

"We are still in process of confirming the nationality," Prime Minister Junichiro Koizumi told reporters who asked him if the submarine was Chinese.

"Once we confirm it, we will disclose it," said Koizumi, who earlier called the incident "regrettable."

A Defense Agency spokesman said Japan had between Friday and Monday spotted two Chinese ships near southern Japan -- one designed to rescue submarines and the other to tow wrecked ships.

The Chinese ships were in international waters some 1,000 kilometersmiles) south of Tokyo, the spokesman said.

Asked if the Chinese ships had a link to Wednesday's submarine incident, the spokesman said: "We don't know."

Japan wants the submarine to surface and show its flag but has not given orders to attack the vessel as it is in international waters, the Defense Agency spokesman said.

Hosoda said the submarine was found near Okinawa's Sakishima island chain, which lies close to islands disputed between China, Japan and Taiwan -- known as Senkaku in Japanese and Diaoyu in Chinese.

The only other time Japan has ordered such a hunt was in March 1999, when two suspected North Korean ships were spotted off its coast. A plane dropped 12 bombs as a warning.

Okinawa hosts about 65 percent of the more than 40,000 US forces in the country.

China feels deep resentment about Japan's refusal to admit its atrocities during its occupation from 1931 to 1945 -- a feeling reinforced by Koizumi's regular visits to a Tokyo shrine that honors the war dead including convicted war criminals.

A fact-finding mission to China by six Japanese lawmakers recommended Wednesday slashing aid to China, noting that Beijing tolerated anti-Japanese sentiment and was making its own loans to other countries.

Japan has also said China may be planning new fields in a disputed gas project around the maritime boundary between the countries, which are both major energy importers.

A Japanese defense study reported Monday envisaged three scenarios for a possible Chinese attack on Japan, including one focused on Okinawa in which China would try to stop US forces from helping Taiwan in the event of a conflict over the island.

China voiced anger over Japan's "Cold War mentality" and said it was not certain that President Hu Jintao would meet Koizumi when they attend a regional meeting in Chile this month.

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