The release of 65 alleged Taliban fighters in Afghanistan poses serious security concerns and sets back the rule of law, NATO head Anders Fogh Rasmussen said Thursday.

"I am gravely concerned by the decision," he said in a statement, noting that the released prisoners were alleged to have killed and wounded Afghan civilians, Afghan security forces and US-led NATO forces.

Rasmussen said the decision appeared to have been "based on political calculations and... is a major step backwards for the rule of law in Afghanistan and poses serious security concerns."

The Afghan government has "to uphold the rule of law and to ensure the security of the country," he said, calling on Kabul to make sure those released do not pose a further threat.

The release has sparked a furious US reaction, with relations between Kabul and Washington already badly strained by President Hamid Karzai's refusal to sign an accord allowing some American troops to remain in Afghanistan after NATO's withdrawal this year.

The US embassy in Kabul said the prisoner release was "deeply regrettable."

"The Afghan government bears responsibility for the results of its decision. We urge it to make every effort to ensure that those released do not commit new acts of violence and terror."

Karzai has called Bagram prison where the men were held a "Taliban-producing factory" and alleged that some detainees were tortured into hating their country.

Some analysts believe he hopes the releases could help kick-start moribund peace talks with the Taliban, who were ousted from power in 2001.