America has been secretly releasing high-level Taliban prisoners from a top security military prison as part of negotiations with insurgents.
Up to 20 have been freed from the jail at Bagram air base in the past two years after giving assurances that they would reconcile with the Afghan government.
The "strategic release" programme has allowed the US to use prisoners as bargaining chips when trying to reach local deals with insurgents.
Officials admitted the scheme was risky and difficult to police. They would not say whether any of those released had resumed attacks on Nato or Afghan forces. "Everyone agrees that these are bad guys. But the benefits outweigh the risks," one told the Washington Post.
A spokesperson for the US embassy in Kabul, said the programme was two years old and "rarely used". He said: "Fewer than 20 detainees have ever been released under this programme, and the decision to release a detainee takes into account whether they pose any further security threat."
Prisoners have become a significant hurdle to embryonic peace contacts aimed at finding a political settlement to the conflict.
Contacts in Qatar appeared to founder earlier this year when Taliban negotiators pulled out, saying that America had broken a promise to transfer five leaders from Guantanamo Bay to looser custody in the Gulf state. Unlike Bagram, releasing men from Guantanamo requires congressional approval.
Senior prisoners have in the past been transferred to Afghan custody only to be released in murky circumstances. One, handed into Afghan custody from Guantanamo in 2007 was freed, then rejoined his Taliban comrades and quickly assumed a leadership position.
