An explosion was heard near a US Army base south of Tokyo late Monday and police suspected an attempted attack on the base, Kyodo News agency reported.
The blast was in the area of the US Army's Camp Zama, Kyodo said. There were no reports of any injuries or damage.
Police found parts of a "launch pad" near the base, Kyodo said, and investigators suspected a "guerrilla attack."
No further details were immediately available.