The bodies of the two women were found by villagers on the outskirts of the city of Bannu.

A note found on the bodies accused the women of "acts of obscenity", a term that usually refers to prostitution.

The region is a known base for militants who want to impose their interpretation of Islamic law.

Police said the women were travelling in a three-wheeled vehicle when masked and armed men overpowered them and bundled them into a car.

Senior district police officer Dar Khattak told Reuters news agency it was the first time militants had directly targeted and killed women in the region.

The note read: "We have started doing this to end obscenity in the area."

Music and movie shops in the region have also been targeted by militants.

Militant attacks in the north-west have increased since the army ousted radical Islamists from the Red Mosque in the capital, Islamabad, in July. More than 100 people were killed in the operation.