PESHAWAR: Police say four boys were killed while playing with an abandoned explosive device that went off in northwest Pakistan.

The children were playing near a pond in the garrison town of Kohat with what officials describe as a ''shell'' apparently from a mortar.

Senior government official Khalid Khan Umarzai says Wednesday's blast appeared to be an accident.

Militant and criminal gangs are active in the Kohat region, and all sorts of weapons, including landmines and grenades, can be found in the area.

Kohat also lies near the tribal regions bordering Afghanistan, where al-Qaida and Taliban fighters have bases.