In Ferozkoh, the capital of the province, a car hit a roadside mine that killed a husband, wife and their two children, Karimi said.
No group including the Taliban has claimed responsibility for the blast.
On Wednesday, four civilians were killed in a roadside mine blast in the Kharistan village of Ferozkoh.
A day before three civilians were shot and killed by unknown gunmen while they traveled on the Ghor-Herat highway.
Local officials have blamed the Taliban for the shooting, but the group has not yet commented.