Koichiro Gemba
© Agence France-PresseJapan's foreign minister Koichiro Gemba
Tokyo - Japanese leaders reacted angrily on Friday after the police on Okinawa said an American Air Force serviceman was suspected of breaking into an apartment while drunk and punching a 13-year-old boy, just weeks after two American sailors were accused of raping a woman on the same island.

Japan's foreign minister, Koichiro Gemba, called the suspected attack "outrageous," and he said hitting a boy was "completely unforgivable."

The assault took place early Friday morning, the police said. The airman was apparently in violation of a curfew imposed just last month by the American military on all of its roughly 50,000 military personnel in Japan following the rape accusation. The police did not release the name of the 24-year-old airman, who was in the hospital after apparently falling from a third-story window.

The back-to-back episodes have stirred outrage on Okinawa, the southern island that hosts three-quarters of the American bases in Japan. The episodes also threaten to complicate ties between the United States and its closest Asian ally at a time when both nations are trying to work together to face an increasingly assertive China.

The episodes have added to the increasingly vocal opposition on Okinawa to carrying what many residents see as a disproportionate burden in hosting so many of the American troops in Japan. Japanese officials fear Okinawan anger could grow strong enough to disrupt their nation's overall security alliance with the United States, on which Japan has relied for its defense since the end of World War II.

Okinawan opposition has already blocked a 16-year-old deal to relocate a United States Marine Corps airfield to the island and forced the Marines to place restrictions on flights by a new aircraft, the tilt-rotor Osprey. Japanese leaders have been trying to convince Okinawans that they are working to lighten the island's base burden, while also reassuring the Americans that they remain committed alliance partners.

The governor of Okinawa, Hirokazu Nakaima, warned that the suspected actions by American servicemen threatened the entire United States-Japan alliance.

"You can only conclude that they are fracturing the alliance," Mr. Nakaima was quoted as saying by the daily newspaper Asahi Shimbun.

American officials said they would cooperate with the investigation.

"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," the American ambassador to Japan, John V. Roos, said in a statement.

Crimes by American military personnel are an emotional issue on Okinawa, especially since the rape of a girl by three servicemen in 1995. They top a list of Okinawan complaints about the bases that also includes noise and pollution.

The police said the American airman was suspected of entering the apartment around 1 a.m. Friday as two boys inside were sleeping. The American woke them up and punched one of them in the face, the police said, before kicking in a television set and then trying to flee through the third-story window.

They said the American had apparently been drinking at a bar on the ground floor of the same building, where he began shouting and acting violently before going upstairs to the apartment. The police said the door of the apartment, which was rented by a 41-year-old woman, was unlocked, as is still common in parts of low-crime Japan.

It was not immediately clear why the American airman would have gone to the apartment.