A British aid worker who died in a failed US operation in Afghanistan may have been killed by a grenade detonated by her rescuers and not by her kidnappers, Prime Minister David Cameron said on Monday.

Cameron said a full investigation would now be launched into the death on Friday of Linda Norgrove, 36, who was abducted with three Afghan colleagues on September 26 in eastern Afghanistan bordering Pakistan.

"Linda could have died as a result of a grenade detonated by the task force during the assault. However this is not certain and a full US-UK investigation will now be launched," Cameron told a news conference at Downing Street.

Cameron said the commander of US forces in Afghanistan, General David Petraeus, had contacted his office early Monday "to inform us that in the review of the rescue operation, new information had come to light about the circumstances surrounding Linda's death.

"General Petraeus has since told me that that review has revealed evidence to indicate that Linda may not have died at the hands of her captors as originally believed."

"Obviously he deeply regrets what has happened."

The announcement came a day after Western officials in Afghanistan had insisted that Norgrove was likely killed by a suicide vest held by one of the kidnappers.

Norgrove was working for US development group DAI when she and three Afghan staff were captured while travelling in Kunar province, a hotbed of Taliban activity in eastern Afghanistan.

US forces launched the operation late on Friday.

Cameron said the original decision to launch the operation "was not an easy one" and was taken "with my full support".

"Linda's life was in grave danger from the moment she was taken. Those on the ground and in London feared she was going to be passed up the terrorist chain which would increase further the already high risk that she would be killed," he said.