An "improvised launch device" was found outside the United States' Yokota Air Force base in Tokyo on Thursday night but there were no injuries and no damage, a U.S. military spokesman in Japan said.

"There were no injuries and thus far we have found no damage or impact points here on base," the spokesman added in a statement on Friday.

Residents near the base in western Tokyo called police last night to report hearing explosions, national broadcaster NHK said, adding that police found several metal pipes pointing at the base in a field some 300 meters (yards) away.