Nine persons were killed and several others injured when a police van transporting prisoners was targeted in a roadside bomb attack in Pakistan's restive North West Frontier Province on Thursday.

The van, which was taking prisoners from a jail to court, was attacked near a bridge in the Bannu area of NWFP, police officials told TV channels. Most of the dead were believed to be policemen who were escorting the prisoners.

Several other occupants of the van were injured in the blast.

Additional Superintendent of Police Waqas Ahmed told reporters it was believed that the van was targeted with an improvised explosive device. Some TV channels, however, reported that a suicide bomber might have carried out the attack.

No group has claimed responsibility for the attack.