A court in Japan's southern prefecture of Okinawa has sentenced a US Air Force member to five years in prison for sexually assaulting an underage girl last December.

The Naha District Court handed down the ruling on Friday against 25-year-old Brennon Washington, who belongs to Kadena Air Base in Okinawa.

He allegedly approached the girl with malicious intent at a park before forcibly taking her to his home, where he sexually assaulted her. The two were not known to each other.

The focus of the trial was whether the defendant was aware that the girl was under the age of 16, and whether the sexual act was consensual.

Washington pleaded not guilty, while prosecutors sought a seven-year term.

In the ruling, the presiding judge said the girl's claim that she used gestures and other means to tell the defendant her age is credible, based on security camera footage.

The judge said the girl had also told the defendant to "stop" in English, so he must have been aware that she may not have given her consent.

The judge described the case as extremely malicious, saying the defendant continued the assault even after she clearly rejected him.

The case was one of a series of alleged sex crimes by US service members in Okinawa that were belatedly disclosed in June this year.

The central government and Okinawa police were criticized for failing to report the cases to the prefectural government on the grounds of protecting privacy.

The controversy led to the creation in July of a new information-sharing system for sexual assault cases. Okinawa police now inform the prefecture when a US service member is arrested or when such a case is referred to prosecutors.

The prefecture hosts about 70 percent of the US military facilities in Japan.