A UN statement says the explosions injured 50 people and destroyed many houses, causing "desolation" in the heart of Congo's third largest city.
It says "official sources" determined that a lightning bolt on Friday hit the munitions depot of an army barracks outside Mbuji-Mayi and exploding shells hit homes some 6 kilometers away.
UN mission chief Martin Kobler said he had instructed the UN office in Mbuji-Mayi to provide support to local authorities, and he commiserated with victims.