Pakistani police have arrested an imam accused of planting burnt pages of the Koran in the bag of a Christian girl accused of blasphemy, officials say.

The girl was detained two weeks ago near the capital Islamabad after an angry mob demanded she be punished.

However, a witness has testified that imam Khalid Chishti put the pages in the bag himself, local media say.

The case of the girl, named as Rimsha, who is said to be about 14, has sparked international condemnation.

A report by a government-appointed medical board seen by the BBC suggests the girl has a mental age of less than 14.

Earlier this week, a court extended Rimsha's detention at a maximum-security prison by a further two weeks.

Her father has said he fears for his daughter's life and for the safety of his family. He has called on Pakistani President Asif Ali Zardari to pardon her.

Rimsha's parents have been taken into protective custody following threats, and many other Christian families have fled the neighbourhood.

Pakistan's strict blasphemy laws are often used to settle personal vendettas, correspondents say.

Last year two leading politicians were assassinated after speaking out against the legislation.