A bomb has ripped through a van in Pakistan's northwestern city of Peshawar, killing at least seven people and wounding more than 20 others, police have said.

Initial reports indicated that 22 people were injured in the blast on Wednesday, Tahir Ayub, a senior police official told the AFP news agency.

Officals told Al Jazeera that 40kg of explosives had been used in the blast, and the target was a van belonging to Pakistan's air force.

Officers said it was not immediately clear whether the bomb was planted in the van or at the roadside.

Peshawar is the main city in Pakistan's restive northwest bordering Afghanistan, where Taliban and al-Qaeda fighters have strongholds.

Source: Al Jazeera and agencies