Japan's Defense Minister Kihara Minoru says US forces in the country are conducting basic flights of Osprey transport aircraft to help crews recover their skill levels.

Kihara spoke to reporters on Friday, one day after flights of the tilt-rotor aircraft resumed at the US Marine Corps Futenma Air Station in Japan's southwestern prefecture of Okinawa.

The aircraft were grounded after a crash off the southwestern Japanese prefecture of Kagoshima in November. All eight US military personnel on board were killed.

Kihara said he understands that aircraft are conducting basic flights after maintenance and crew training aimed at preventing future accidents.

He also said he and US Secretary of Defense Lloyd Austin reaffirmed in phone talks that ensuring the safety of flights is the top priority for both Japan and the United States.

Kihara pledged that the two countries will continue to work closely together and take all necessary safety steps.