A United Nations document recently seen by the BBC says the Taliban are intensifying their hunt for people who worked for, and collaborated with, Nato and US forces.

And the giant stores of biometric data collected by both the US military and the Afghan government could, some argue, pose a risk to those facing reprisals.

Brian Dooley, a senior adviser to activist group Human Rights First, told the BBC's Tech Tent podcastthat while very little was definitively known, "a very educated guess would say that [the Taliban] either has or is about to get their hands on an enormous amount of biometric data".

Using handheld devices called HIIDE (Handheld Interagency Identity Detection Equipment), soldiers like Mr Kiernan would add the details of Afghans to a US biometric store.

He said it was useful in identifying bomb-makers, while it was also used to confirm the identities of contractors and locals working with the US military.

The original military ambition was to cover 80% of the population (25 million people) on the system, although the actual figure achieved is thought to be much less.