The Taleban say they are delighted that Mullah Abdullah has been released
The Taleban have confirmed that a former Guantanamo detainee has become their senior military commander in southern Afghanistan.
Taleban sources told the BBC that Mullah Abdullah Zakir had led the fight against British forces stationed in Helmand province.
They say he now leads the insurgency in that province and across the south.
Mullah Abdullah was held in Guantanamo until December 2007. He was then handed over to the Afghans and later released.
Last month, UK government officials told the BBC he was now closely involved in planning attacks on British and other Nato forces in Afghanistan.
The officials said Mullah Abdullah was operating with impunity from the Pakistani city of Quetta.
'US not winning'
The BBC's Martin Patience in Kabul says senior Taleban officials say Mullah Abdullah Zakir's release after six years is a huge success for the movement.
Mullah Abdullah is originally from Helmand province, where the majority of British troops are based.
Increasingly the militants are relying on suicide attackers and roadside bombs to target foreign and Afghan forces, often to deadly effect.
The Pentagon says more than 10% of 520 Guantanamo inmates released so far have returned to what it calls terrorism.
It says this complicates efforts to release and repatriate those still being held.
US President Barack Obama has long vowed to close the controversial Guantanamo Bay detention centre.
Earlier this week the most senior American commander in the country, Gen David McKiernan, said that the coalition was not winning in large parts of southern Afghanistan.
In the coming months the US intends to deploy thousands of additional US forces to bolster UK troops in their fight against the resurgent Taleban.