Washington: Barack Obama's administration believes that at least 12 detainees released from the prison at Guantanamo Bay, Cuba, have launched attacks against US or allied forces in Afghanistan, killing about a half-dozen Americans, according to current and former US officials.

In March, a senior Pentagon official, Paul Lewis, made a startling admission to lawmakers when he acknowledged that former Guantanamo inmates were responsible for the deaths of Americans overseas.

Guards and detainees in Guantanamo's Camp 4 detention facility in 2009. Credit:AP

Most of the incidents were directed at military personnel, but the dead also included one American civilian: a female aid worker who died in Afghanistan in 2008. Officials, who spoke on the condition of anonymity, declined to give an exact number for Americans killed or wounded in the attacks, saying the figure is classified.

Lewis's statement drew scrutiny on Capitol Hill, where some lawmakers see the violence against Americans as further evidence that the president's plans for closing the prison are misguided and dangerous. They also describe the administration's unwillingness to release information about the attacks as another instance of its use of high levels of classification to avoid discussion of a politically charged issue that could heighten political opposition to its plans.