Story highlights Japanese defense minister says officials are lodging protests with U.S. authorities

The two 23-year-olds are accused of raping a Japanese woman on Okinawa

Tensions over the U.S. military presence on Okinawa have boiled over in the past

The U.S. government is "extremely concerned" by the allegations, ambassador says

Japanese officials expressed outrage after two U.S. sailors were arrested over accusations that they raped a woman on the island of Okinawa, where the American military presence has generated long-simmering resentment.

Defense Minister Satoshi Morimoto on Wednesday called the alleged rape "vicious and mean" and said Japanese authorities were lodging protests with the U.S. government and military, as well as demands for better preventive measures.

Okinawa Gov. Hirokazu Nakaima, who met with Morimoto over the matter, said it went "beyond madness" that the alleged attack took place two months after a U.S. Marine was arrested over accusations he assaulted and molested a woman in Naha, the capital of Okinawa.

Police in Okinawa identified the detained sailors as U.S. Navy Seaman Christopher Daniel Browning and Petty Officer Skyler Dozierwalker of Naval Air Station Joint Reserve Base Fort Worth in Texas.

The two men, both 23, are alleged to have raped a Japanese woman in the early hours of Tuesday morning, leaving her with an injury to her neck, police said. They were taken into custody later that day.

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Tensions over the American military presence on Okinawa have boiled over before. Many residents were incensed by the rape of a 12-year-old Japanese girl in 1995 by three U.S. military personnel. And allegations that a Marine raped a 14-year-old girl caused a furor in 2008, although the girl decided not to pursue charges.

About half of all U.S. military personnel in Japan are stationed in Okinawa.

The U.S. government is "extremely concerned" by the allegations against the two service members, U.S. Ambassador to Japan John Roos said in a statement Tuesday.

"We are committed to cooperating fully with the Japanese authorities in their investigation," he said.

Japanese Vice Foreign Minister Shuji Kira on Wednesday summoned Roos over the case, urging the enforcement of stricter discipline and preventive measures.

Morimoto, the defense minister, suggested that if the Japanese protests about the matter didn't prove effective enough, he could raise the issue with U.S. Defense Secretary Leon Panetta.

Opposition to the presence of U.S. troops in Okinawa runs so deeply that it contributed to the resignation of former Prime Minister Yukio Hatoyama in 2010. He had promised to move a U.S. base but later announced that it would stay. His critics said at the time that he gave in to U.S. pressure, and his governing coalition broke up.

As well as the cases of misconduct by U.S. troops, some Okinawan residents have complained about issues such as environmental and noise pollution from the American presence.

There is also concern in Okinawa about the U.S. deployment of the Marine Corps MV-22 Osprey , a controversial tilt-rotor aircraft that can take off and land like a helicopter but fly like an airplane.

Doubts about the Osprey's safety have been fueled by two crashes -- one in Morocco and one in Florida -- earlier this year.