The U.S. Marine, 38-year-old Tyrone Hadnott, based at Camp Courtney on the island, was arrested on Monday on suspicion of raping the schoolgirl when the two were in a car on Sunday.

Japanese media said he had denied raping the girl but acknowledged forcing her to kiss him.

"It is unforgivable," Prime Minister Yasuo Fukuda told a parliamentary panel in his first public comments on the latest incident on Okinawa, host to a huge U.S. military presence.

"It has happened over and over again in the past and I take it as a grave case."

Defence Minister Shigeru Ishiba expressed anger, adding: "This will have a big impact on future U.S-Japan relations."

In 1995, the rape of a 12-year-old Japanese schoolgirl by three U.S. servicemen sparked huge protests calling on the U.S. military to leave Okinawa, where residents have long resented crime, noise and accidents they blame on the U.S. presence.

But diplomatic experts said such political fallout could be limited this time if the two governments were careful.

"I don't see that there is the sort of dry kindling there for this to light," said Derek Mitchell, a senior fellow at Washington-based Center for Strategic and International Studies.

"I think the alliance is on much more solid ground."

The 1995 rape case coincided with bitter trade talks on Japan's auto market as well as doubts about the significance of the alliance after the end of the Cold War.

Japan is home to some 50,000 U.S. troops, under a security alliance that is a pillar of Japan's post-war diplomacy.

OKINAWA REACTION KEY

Both U.S. and Japanese authorities want to prevent a rerun of 1995 but much depends on the reaction in Okinawa, analysts said.

Okinawan officials have expressed outrage and local assemblies were set to issue formal protests, media said.

"Local residents and labour unions are becoming very active," one Okinawa journalist said. "There will be demonstrations and I think they could be rather large."

However, unlike in 1995 when the Okinawa governor was a staunch critic of the bases, the current governor was elected with ruling party backing and is inclined to support Tokyo on U.S. military issues.

"Mr Fukuda is, I think, making every effort to keep this incident from being too problematic," said Takeshi Inoguchi, an international relations expert at Chuo University in Tokyo.

U.S. officials have responded quickly to mitigate fallout from the case, which comes as Tokyo tries to persuade Okinawa residents to accept a plan to relocate the Marine's Futenma Air Station from the densely populated central Okinawa city of Ginowan to the coastal city of Nago.

"Obviously, the U.S. military is cooperating with the Okinawan authorities who are investigating this," Pentagon spokesman Bryan Whitman said in Washington, adding, however, that the Marine was presumed innocent until proven guilty.

"I wouldn't tie our long-term strategic relationship with Okinawa, that part of the world to this particular incident."

The Futenma move is part of a broader plan to move some 8,000 Marines from Okinawa to Guam.