At least seven people have been killed after a car bomb targeted a prison bus in Pakistan, police said.
Six policemen and a civilian died in the morning blast near Bannu town in North West Frontier Province (NWFP).
Police told the BBC Urdu service that the device was in a vehicle parked on a bridge and was detonated by remote control as the prison van approached.
Bannu is close to the border with Afghanistan, where Islamist militants have been attacking security forces.
Police said no prisoners were on the bus at the time of the blast, at about 0900 (0300 GMT) in the garrison town, 250km (155 miles) south-west of the Pakistani capital Islamabad.
A local police superintendent told the BBC: "Six policemen and a civilian have been killed. The prison bus was on its way to the jail to pick up prisoners for their court hearings.
"Near the bridge on Kurram River, an explosive-laden vehicle was already parked. When the bus approached the vehicle, the explosives were detonated by a remote control."
AFP news agency reported that the force of the blast had caused the prison van to plunge into the river below.
No one immediately claimed responsibility for the attack.
Security has deteriorated sharply in recent weeks in NWFP, which the Afghan government and the US say is a haven for pro-Taleban militants and al-Qaeda.