Japanese officials say an explosion at a storage site for unexploded ordnance at a U.S. military base on Japan’s southern island of Okinawa injured four Japanese soldiers.

An explosion occurred at the northern section of a munitions storage area on Kadena Air Base, Okinawa, according to the Air Force.

Four Japanese Ground Self-Defense Force members were injured Monday after a piece of military ordnance went off at the U.S. Air Force base.