Deputy provincial chief police Abdul Ali Zamani said that the explosion took place in the district of Qadis in Badghis province.
He says a van with the civilians hit the roadside bomb on Wednesday morning, raising the possibility that it was detonated by remote control.
Zamani blamed the Taliban for the attack though no has yet claimed responsibility for the bombing.
Last month the Taliban proclaimed their spring offensive and a week later, launched a massive attack in the capital, Kabul, which killed 64 people.