A passenger bus was transporting people from Botswana, a landlocked country in Southern Africa, to Moria, a town in Limpopo
The incident in the West African country's Mopti region happened late on Monday night.
"The first report puts the toll at 20 killed and 63 rescued by emergency services," the government said in a statement.
"According to the initial stages of the investigation, a technical failure relating to the brake system could be the origin of this dramatic accident," it said.