A submarine prompted Japan's naval forces to go on a rare alert after it intruded into Japanese waters. Defense officials think the intruder was a Chinese nuclear-powered submarine.
For only the second time since the end of World War II, Japanese naval forces went on alert. The action was prompted when a submarine was spotted in waters of the Okinawan island chain in southern Japan.
Prime Minister Junichiro Koizumi told reporters that the Maritime Self Defense Force is still tracking the suspect submarine.
Mr. Koizumi says the government has not yet determined what country owns the submarine and he brushes off reporters' questions about whether Japan would protest to China, saying there would be no such action until the identity of the submarine is determined.
Japanese news media report that a sonar buoy determined that the vessel's propeller noise was consistent with that of a nuclear-powered attack submarine used by the Chinese Navy.
Government officials say the sub was in Japanese waters for about three hours.
Defense Agency Director General Yoshinori Ono says he invoked the Self-Defense Forces law to order the security action after getting approval from the prime minister.
Mr. Ono says no shots were fired at the submarine but Japanese aircraft demanded that it surface and show its flags or leave the country's waters. He adds that the maritime forces will do what is necessary to maintain national security.
Suspicions immediately focused on China and North Korea. But defense sources and analysts say North Korea probably does not have submarines capable of operating so far from its shores.
The submarine incident comes after South Korea's Navy said a North Korean patrol boat briefly strayed into South Korean waters in the Yellow Sea for about 40 minutes Tuesday evening.
Military officials in Seoul say the boat apparently was pursuing a Chinese fishing vessel. South Korean officials say three warning messages were sent to the North Korean boat, which replied it was attempting to keep control of ships in its waters.
Last week, South Korea's navy fired warning shots in the same area after three North Korean patrol boats crossed the frontier. North Korea later accused the South of a grave provocation that could lead to a clash.
Also Wednesday, South Korean Foreign Minister Ban Ki-moon expressed optimism about the prospects for another round of multilateral talks about North Korea's nuclear weapons development.
Mr. Ban, speaking at a forum, said a fourth round of talks could follow this week's discussions in Pyongyang between Japan and North Korea.
A delegation from Tokyo is in the North Korean capital hoping to learn more about Japanese kidnapped decades ago by North Korean agents.
Three rounds of talks - involving both Koreas, the United States, China, Japan and Russia - have not resulted in any breakthroughs.