According to police officials, the van was targeted when it was on routine patrolling of Dera Ismail Khan, a gateway to Pakistan's troubled tribal region bordering Afghanistan.
Two women and a child were also among 19 those injured, police said.
The bomb which had been planted near the circular road, General Bus stand, weighed between five to seven kilogrammes, police officials were quoted as saying by Express Tribune.
All injured were shifted to a nearby hospital. Police reached the site and collected evidence after cordoning off the area.