At least 34 civilians were killed when a roadside bomb hit a passenger bus on Herat-Kandahar highway on Wednesday, authorities said.
Seventeen others were wounded in the bombing, said Sediq Sediqi, a spokesman for the president.
There was no immediate claim of responsibility for the bombing.
Sediqi blamed the Taliban for the violence.
It comes a day after the United Nations said more than 3,800 civilians were killed or wounded in the first six months of 2019