Story highlights "It's an understatement to say I'm very upset," U.S. ambassador says

A U.S. airman is alleged to have broken into an apartment and hit a 13-year-old boy

The alleged attack is "outrageous," the Japanese foreign minister says

Two U.S. sailors were arrested last month over accusations they raped a local woman

Compounding the American military's difficulties on the Japanese island of Okinawa, a U.S. airman is under investigation over allegations he broke into a local family's home early Friday and assaulted a teenage boy before jumping off a third-floor balcony.

The incident is likely to further deepen resentment among Okinawan residents about the significant U.S. military presence on the island. The situation was already extremely delicate following the arrest last month of two U.S. sailors accused of raping a local woman

That case prompted angry protests from Japanese officials and local residents. The U.S. military responded by imposing a nighttime curfew on its thousands of troops in the country -- a restriction the airman appears to have disobeyed Friday.

According to Okinawa police, the suspect is alleged to have broken into the family's apartment in the village of Yomitan around 1 a.m. Friday, hit a 13-year-old boy who was in bed and damaged a television set. The boy was left with an injury to his cheek.

The airman suffered "possible broken bones and internal injuries" after jumping from the apartment's balcony and has been admitted to a military hospital on the nearby U.S. Air Force base of Kadena, U.S. military officials said.

Japanese Foreign Minister Koichiro Gemba called the incident "outrageous," noting that it took place despite the U.S. military curfew. He said the Japanese authorities would lodge a complaint with the United States.

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The U.S. ambassador to Japan, John Roos, said he was also appalled by reports of the incident.

"Let me be absolutely clear: I am very upset -- it's an understatement to say I'm very upset," he told reporters in Tokyo on Friday, expressing concern about the well-being of the Japanese boy who was allegedly attacked.

He stressed that the U.S. military forces in Japan are "undergoing a complete review of the liberty policies and other policies that will minimize, if not eliminate, any such incident in the future."

"It is incredibly unfortunate that the purported actions of a few reflect badly on thousands of young men and women here in Japan, away from their homes, that are here for the defense of Japan," he said.

American military officials on Okinawa, which lies south of the main Japanese islands, were trying to deal with the fallout from the situation.

"It is extremely regrettable when an alleged incident like this occurs," said Col. Brian McDaniel, vice commander of the 18th Wing of the U.S. Air Force, which occupies the Kadena base, the largest American military installation in the Asia-Pacific region. "We are fully cooperating with Okinawan authorities on this investigation to ensure justice is served."

Maj. Christopher Anderson, the head of public affairs for the 18th Wing, said he had met with the mayor of Yomitan on Friday.

"This isn't how we want our people to conduct themselves," he said of the airman's alleged behavior.

Japanese and U.S authorities declined to disclose details of the airman's identity Friday, other than that he was assigned to Kadena.

The issue of violent crimes by U.S. troops in Japan has divided the two countries for decades. It came to a peak in 1995 when a U.S. sailor and two U.S. Marines were convicted of raping a 12-year-old girl. Tens of thousands of Okinawans took to the streets at the time demanding that the United States leave the island.

Relations between the U.S. military and the people of Okinawa have also been strained in recent months over the U.S. Marine Corps' deployment of MV-22 Osprey tilt-rotor aircraft to a base on the island. Some Okinawa residents are concerned because the Osprey has had a reputation for crashing.