Japan Self-Defense Forces (SDF) members at the facility at Kadena Air Base were trying to wipe rust off items at the facility to re-assess if they were bombs, said local fire department official Akira Kamiunten.

During that process there was an explosion that injured four male SDF members who were rushed to hospital, he told AFP.

Jiji Press and other local media said none of the injuries was life-threatening.

A defence ministry spokesman also confirmed reports of an explosion at Kadena Air Base, located on the main island of the southern region of Okinawa.

During World War II, Japan used Okinawa as a buffer to slow US forces.

More than a quarter of the main island's civilian population died in the 1945 Battle of Okinawa.

US occupation only ended there in 1972, under a mutual treaty that left American bases in place.

Today, Okinawa contains 70 percent of the land used for US bases in Japan, and more than half of the 50,000-strong US troop presence.