By Chris Hogg
BBC News, Tokyo
Japan's prime minister has described the alleged rape of a 14-year-old girl by a US marine as "unforgivable".
Okinawans have protested against previous attacks by US marines
Marine Tyrone Hadnott, 38, is being questioned in relation to the alleged assault on the island of Okinawa.
He denies raping the girl in his car on Sunday night, but police say he has admitted forcing her to kiss him.
Prime Minister Yasuo Fukuda condemned the alleged assault in parliament, complaining that this kind of grave incident had happened repeatedly.
In the past, assaults by US marines have fuelled local anger at the presence of many thousands of foreign soldiers on Japan's southernmost island.
Twelve years ago, the rape of a 12-year-old girl on Okinawa by three US servicemen led to street protests against the presence of the American military there.
Similarly, the most recent incident could damage the US-Japan military alliance.
Mr Fukuda said his government would do what it could to try to prevent a repeat of such incidents in the future.
Sunday's alleged assault has also brought condemnation from Okinawa's governor and other local politicians.
The US state department said it was taking the allegations very seriously and pledged to work closely with the Japanese.
About 50,000 US troops are stationed in Japan, most of them on Okinawa.
Last year, 46 US military personnel were arrested in Okinawa in connection with criminal cases.
That figure, though, represented a 50% drop compared to the number of arrests made on the island five years ago.