Mohammad Naser Mehri, spokesman for the provincial governor in Farah province, said Sunday that more than 40 others were wounded in the accident.
The bus was on its way from western Herat toward the capital, Kabul, when the accident took place, Mehri said, adding around a dozen of the wounded are in critical condition.
Traffic accidents are not unusual in Afghanistan; annually hundreds of people die in traffic accidents on highways across the country mostly as the result of unpaved roads, high speeds and carelessness while driving.