Sulaiman Muradi, a spokesman for the provincial governor, said the explosion happened around 2 p.m., local time, in Derqad port in Khwajah Bahauddin district.
He said it was unclear whether the victims were Afghan police officers or civilians. A police Ranger vehicle was the target of the explosion.
No group has claimed responsibility for the attack, Khaama Press.
Meanwhile, the Taliban released the four children and driver of abducted Khandahar province legislator Fariba Kakar.
She and her children were abducted Sunday while traveling to Ghanzi province by Taliban militants. The Taliban demanded the release of four prisoners in exchange for Kakar, Khaama Press said.