The incident happened at about 9:30 a.m. local time (0430 GMT) in Lakki Marwat district of the province when a group of children, while playing in the nearby mountainous area, found an unexploded mortar shell, police sources told Xinhua.
The device went off as the children began tampering with it, resulting in multiple casualties, including five fatalities, the sources added.
Rescue teams shifted the injured to a nearby hospital for medical treatment.
The sources said an investigation has been launched to trace the origin of the explosive device.