A defence ministry spokesman said they had received reports of an explosion at the Japan Self-Defense Forces (SDF) facility inside Kadena Air Base in the southern Japanese region.

Jiji Press and other local media said four injuries had been reported but none were life-threatening.

Public broadcaster NHK said, citing unnamed defence ministry sources, that the explosion may have occurred at a temporary storage site for unexploded bombs, with officials trying to confirm the situation.

"We've heard there was an explosion at the SDF facility and also heard there were injuries but we don't have further details," Yuta Matsuda, a local official of Yomitan village in Okinawa, told AFP.