The city of Mbuji-Mayi "was ravaged with more than 20 dead, around 50 wounded and many destroyed houses" yesterday, MONUSCO said in a statement.
A government spokesman gave an initial toll of at least five dead from the blast triggered by a lightning strike on the military munitions dump at the Brigade army base, near the city's main market.
The UN said MONUSCO chief Martin Kobler had told the peacekeeping mission's local office to "help and support the local authorities to deal with the situation".
A witness speaking by telephone, who declined to be named, said he had seen the bodies of two people -- a woman and her child -- in the local hospital, as well as several people with amputated limbs.