A US Air Force member charged for sexually assaulting an underage girl in Japan's southern prefecture of Okinawa has pleaded not guilty to the allegations.

The first hearing of the trial for 25-year-old Brennon Washington took place at the Naha District Court on Friday. Washington is stationed at the Kadena Air Base in the prefecture.

The indictment alleges that Washington approached the girl, who was under 16, at a park in the central part of Okinawa's main island in December, then drove her to his home and sexually assaulted her. They did not know each other.

The defendant denied the charges, claiming that he neither kidnapped nor sexually assaulted the girl.

His attorney also argued that the defendant did not approach the girl with the intention of committing indecent acts.

The attorney also insisted that the defendant believed that the girl was 18 years old and the sexual interaction was consensual.

But prosecutors said in their opening statement that the defendant had driven the girl to his home and sexually assaulted her after making it impossible for her to express non-consent.

They also told the court that the girl told her mother about what had happened to her after returning home, and the mother immediately called the police.

In the next hearing scheduled for August 23, the victim and her mother are set to testify in the court.