"The passengers were sprayed with bullets and the vehicle was torched," during an attack by militias near the town of Bandiagara on Friday, the officials said.
"The state has sent security forces to the scene," they said, speaking on condition of anonymity.
An elected official in Bandiagara confirmed the death toll, saying the victims included women and children.
The attack has not been claimed by any of the numerous armed groups active in the West African country.
Mali's transitional government said in a statement broadcast on state TV that 31 people were killed and 17 injured and vowed to do everything necessary to "arrest and punish" the perpetrators.
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