"It was a bomb planted by the Taliban to hit security forces but unfortunately it hit a passenger bus," Farah provincial police spokesman Muhibullah Muhib told AFP.
According to the latest information, the bombing killed 11 people and injured 31.
No terrorist organization operating in the region has claimed responsibility for the explosion.
The explosion happened as the bus passed through Bala Baluk district of Farah at 4:30 a.m. (00:00 GMT), according to provincial governor spokesman Naser Mehri.
The Taliban has been very active in Farah. It started a major attempt to take over the provincial capital in May, causing intense fighting with Afghan forces.
*Daesh (also known as ISIS/ISIL/IS) and Taliban are terrorist groups banned in Russia