Deputy governor Mohammed Rasoolyar says the bombing took place in Zabul province, along a main highway linking the capital, Kabul, with the restive south.
Rasoolyar says two children and an elderly man were also killed in Friday's explosion and that a ninth person was wounded. Police said they are investigating whether the victims were from the same family.
No one immediately claimed responsibility for the explosion, but blame will likely fall on Taliban insurgents.
The south is considered a Taliban stronghold and insurgents there stage near-daily attacks against Afghan troops and also NATO soldiers. Their bombs routinely kill innocent civilians.
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