"It is very regretable. I have always called for discipline to prevent such a case," Koizumi told reporters following news of the incident, which could refuel Okinawa's antipathy against a huge US military presence there.

But he made no commitment to renewed demands from Okinawans that an agreement on the status of US forces in Japan be changed to make it easier for Japanese police to investigate serious crimes committed by US troops.

Japanese media have reported that a 21-year-old American lance corporal was being questioned by police on a voluntary basis over allegations that he had raped a 19-year-old Okinawan woman in late May.

The unidentified marine has admitted to having sex with the woman by "consent" when he went out of a bar with her in the early hours of May 25, the reports said.

A spokesman for the Okinawan prefectural police refused to comment on the case pending the issue of an arrest warrant.

Foreign Minister Yoriko Kawaguchi told the lower house foreign affairs committee, "Honestly, I thought to myself, 'It's happened again.' I strongly hope that this will be the last instance of such a thing."

The incident took place at a delicate time when the US Defense Department is reportedly planning a broad realignment of US troops in Asia that may include moving marines out of Japan.

US Deputy Defense Secretary Paul Wolfowitz has said US forces in Okinawa may undergo some changes but denied a press report that troops in Okinawa might be moved, possibly to Australia.

US Ambassador to Japan Howard Baker expressed regret over the incident when he telephoned Japanese Vice Foreign Minister Yukio Takeuchi late Thursday, a ministry official said.

Baker said that "the type of behavior alleged is completely unacceptable," according to the official.

He has also said that the US military would continue fully cooperating with Japanese police investigations.

Under the Status of Forces Agreement, governing issues of legal jurisdiction relating to US troops stationed here, the US military is not required to hand over to Japanese police criminal suspects among its members, until they are indicted.

But Washington agreed to give "sympathetic consideration" to the immediate handover of suspects in serious crimes, such as murder and rape, after the gang rape of a 12-year-old Okinawan girl by three US servicemen in 1995 sparked off massive protests against US troops and military bases.

The Okinawan chain, strategically located within the striking distance of the Korean peninsula and China, hosts about two-thirds of the 47,000 US troops in Japan.

A string of crimes committed by US soldiers as well as disputes over the ownership and use of the land on which US military facilities sit have made Okinawa residents reluctant hosts -- amid memories of bloody battles with US troops in the closing days of World War II and US military occupation until

WAR.WIRE