Local residents say the woman was targeted because of her brother's long-forgotten secret affair with a woman from the assailants' family.
The incident, which came to light on Thursday when the media got wind of it, took place last week near the remote town of Chaudhwan, some 80km (50 miles) west of Dera Ismail Khan city in Khyber Pakhtunkhwa province.
Police have arrested eight men and are searching for a ninth.
Witnesses say the girl was attacked in broad daylight when she went to fetch water from a pond.
