The bombers were disguised in burqas, the all-encompassing veil worn by conservative Muslim women in parts of Afghanistan and Pakistan, allowing them to get close to the building without arousing suspicion, said police officer Abdullah Khan.

They struck within minutes of each other, with the second blast the bigger and more deadly.

Government official Dilawar Khan Bangash said the attack left 41 people dead and 62 wounded.

The registration point - essentially a small building in a dusty field - may have been hit to persuade people not to have any contact with the local administration or foreign relief groups.

The victims were among around 200,000 people to have left the Orakzai region along the Afghan border since the end of last year, when the Pakistan army began offensive ground and air operations against militants based in the remote, tribally administered region.

AP