The banned Tehrik-i-Taliban Pakistan (TTP) claimed responsibility for the attack in an email sent to journalists.
The police van was on a routine patrol in Sro area of Charsadda’s Shabqadar tehsil when the bomb went off along the roadside, injuring four police officials and five passers-by.
Police and rescue officials arrived at the scene and shifted the injured to tehsil headquarters hospital.
According to hospital administration, three of the injured who were in critical condition were shifted to Peshawar.
Shabqadar tehsil is close to Mohmand tribal region, which is one of seven semi-autonomous tribal regions in the northwest, where Pakistani Taliban and Al Qaeda-linked militants were said to have carved out strongholds.
At least 16 people were killed in March this year when a suicide bomber blew himself up in the premises of a local court in Shabqadar.
The number of attacks in the country has fallen around 70 per cent, due to a combination of the military offensive against Taliban bases along the Afghan border and government initiatives to tackle militancy, but attacks on security and civilian targets continue to occur occasionally.