"We will urge U.S. forces in Japan to tighten discipline and fully implement preventive measures," Hayashi, who serves as chief cabinet secretary, told a press conference.

The case is the latest in a series of violent attacks on women and the accused has been referred to prosecutors, according to an investigative source.

Under the Japan-U.S. Status of Forces Agreement, U.S. military personnel will in principle remain in the custody of the United States until being charged by Japan, unless they are arrested at the scene of a crime.