A girl in Japan's southern prefecture of Okinawa has testified in court that she repeatedly attempted to stop the advances of a US Air Force member accused of sexually assaulting her.

Brennon Washington, stationed at Kadena Air Base in Okinawa, is standing trial for allegedly kidnapping and sexually assaulting the girl in December last year. She was under 16 years old at the time, and they did not know each other.

Washington, 25, is accused of approaching the girl at a park in the central part of Okinawa's main island, then driving her to his home before attacking her.

Washington pleaded not guilty in the first court hearing last month, claiming he believed she was 18 years old and that the sexual act was consensual.

The girl testified on Friday at the Naha District Court, speaking behind a partition to protect her privacy.

She said that she conveyed her age to Washington in English, Japanese and even using finger gestures. She also said that she went to his home out of fear of being harmed if she refused.

The girl denied that the act was consensual, saying she repeatedly told him to stop in both Japanese and English.

She also said that she has since found it difficult to sleep, leave the house and control her emotions, adding that she wants Washington to understand the severity of his crime.

Okinawa Governor Tamaki Denny told reporters on Friday that he will visit the United States from September 8. He said he plans to discuss a series of suspected sexual assault cases involving US service members and other controversial issues, such as the relocation of US Marine Corps Futenma Air Station within Okinawa.