The Norgrove case comes at a critical time for US and Nato forces, with violence on the rise across Afghanistan [EPA]

The commander of US and Nato forces in Afghanistan has said he is "disturbed" by the lack of facts about the death of a UK aid worker killed in attempt to free her from her Taliban captors.

General David Petraeus said in London on Friday the possibility that Linda Norgrove had been killed by a US soldier, and not her captors as initially thought, emerged when commanders studied enhanced video of the operation afterwards and saw a "throwing motion and then an explosion".

"It was disturbing, clearly, not to have the correct facts the morning after the operation was conducted, and to be provided those later, after the task force commander conducted further examination," he said.

He said that when footage had been pulled off a computer hard drive to provide a sharper image, it had become clear that a grenade had been employed.

He did not confirm reports that a member of the elite US Navy SEALs could face disciplinary action for failing to inform commanders that he had tossed a grenade during the operation.

He said a joint American-British investigation would determine the specific facts.

Petraeus told David Cameron, the British prime minister, on Thursday that he would make the investigation of Norgrove's death a "personal priority".

Nato initially had said Norgrove died when her captors detonated a bomb as US forces moved in to free her.

But it later emerged that she may have been killed by a grenade thrown by US troops, after US special forces failed to see that she had broken away from the kidnappers.

The incident comes at a critical moment for US and Nato forces, amid reports that senior Taliban leaders may join negotiations with the government.

Reconciliation efforts

Both the US and Nato pledged on Thursday to support efforts by Hamid Karzai, the Afghan president, to reconcile with elements of the Taliban.

Petraeus admitted on Friday that Western troops have facilitated the safe passage of Taliban leaders to Kabul for talks with the government.

"There are certain ongoing initiatives in that regard," Petraeus said when asked about the state of negotiations with the movement.

"And indeed in certain respects we do facilitate that, given that, needless to say, it would not be the easiest of tasks for a senior Taliban commander to enter Afghanistan and make his way to Kabul if [Nato-led] Isaf were not witting and therefore aware of it and allows it to take place."

Karzai this month launched the High Council for Peace, the latest effort to persuade the Taliban and other insurgents to negotiate an end to the war which has entered its 10th year.

Violence has surged in recent days. Three Nato soldiers were killed on Friday, adding to the 14 other foreign troops killed since Wednesday.