Japan complained on Friday about a "deplorable" incident in which a U.S. serviceman is believed to have assaulted a teenaged boy on the southern island of Okinawa where opposition to U.S. military bases is running high.
The U.S. ambassador to Japan, John Roos, was summoned to the Foreign Ministry over the incident and later said the United States was very upset about it and would cooperate fully in an investigation.
A U.S. servicemen was suspected of breaking in to an apartment to assault the boy early on Friday, media said.
The incident came shortly after the United States imposed a night-time curfew on military personnel after two U.S. servicemen were arrested on suspicion of raping a Japanese woman.
Many people on Okinawa oppose a U.S. base there and disagree with a government decision to allow the United States to deploy Osprey hybrid aircraft on the island despite concern about their safety.
"This happened when a night-time curfew was in place and it is extremely deplorable," Prime Minister Yoshihiko Noda was quoted as saying by Kyodo news agency, referring to the assault on the boy.
"A supposedly impermissible thing has happened."
A foreign man, reportedly a U.S. serviceman from the U.S. Air Force's Kadena base, broke in to an apartment on the third floor of a building and hit a 13-year old boy in the face, Okinawa police said.
The man, who had apparently been drinking, seemed to have fallen out of a third-floor window and was hospitalized, police said.
"We are very upset and we pledge complete cooperation with the government of Japan in getting to the bottom of this and preventing future occurrences," U.S. ambassador Roos told reporters after he was summoned to the ministry.
Such incidents can put a serious strain on relations between the allies although a sharp deterioration in Japan's relations with China recently over a disputed island chain has highlighted the importance for Japan of its alliance with the United States.
Friction over the U.S. presence on Okinawa intensified after the 1995 rape of a 12-year-old Japanese girl by three U.S. servicemen. The case sparked widespread protests by Okinawans, who had long resented the U.S. bases because of crime, noise and deadly accidents.