0259 GMT June 21, 2021
The attack occurred on Saturday evening in the western province of Badghis, according to AFP.
No group has claimed responsibility for the blast but Badghis governor Hessamuddin Shams accused the Taliban of planting the bomb.
Another official from the province Khodadad Tayeb confirmed the toll and said that the bus fell into a valley after it was hit by the bomb.
Saturday's attack came after a series of blasts targeted passenger buses in Kabul this week.
The Daesh terrorist group claimed two back-to-back attacks on buses in Kabul.
Violence has soared in recent weeks as government forces and the Taliban clash in near-daily battles across the rugged countryside, with the militants appearing to focus on capturing new territory and battering checkpoints and bases near Kabul.
The Taliban said on Saturday that they have "captured the district of Deh Yak" in the province of Ghazni, about 150 kilometers south of Kabul.
The authorities said they had only "relocated" their forces from the area.
Ghazni is strategically located on the main road from Kabul to Kandahar, the former bastion of the Taliban in the south. The province sees regular fighting between the two warring sides.
In 2018, the Taliban briefly seized the provincial capital Ghazni, which has the same name as the province, in an all-out attack that left several government buildings torched and destroyed.