The US military has suspended joint operations with Afghan forces because of a collapse in trust after a surge in the number of Americans and other Nato soldiers killed by the men they are fighting alongside or training.
The chief US military officer, General Martin Dempsey, described the sharp rise in "insider attacks" by rogue Afghan soldiers and policemen, which saw four American and two British soldiers killed at the weekend, as "a very serious threat to the campaign" against the Taliban.
American commanders said that joint operations on the ground will be suspended "until further notice" in a dramatic admission that the strategy to shift responsibility for fighting the insurgents to local forces has been deeply compromised by Afghan government soldiers and policemen killing 51 Nato soldiers in 36 attacks this year. At least 12 attacks were carried out last month alone, leaving 15 dead.
The US defence secretary, Leon Panetta, described the attacks as the "last gasp" of a weakened Taliban. But the admission that Nato troops are no longer safe from the forces they are relying on to keep the Taliban at bay after the final US pullout in 2014 is a severe blow to Washington's military plans.
Under the strategy, members of the soon to be 350,000 strong Afghan security forces gain experience patrolling and fighting alongside American and other foreign soldiers. But the killings have led to a collapse in trust.
The US army said it is "not walking away" from Afghan military units and will continue to advise them. But Nato troops will patrol with them only when specific approval is given by a regional commander.
American officials say the insider attacks are carried out by a mix of Taliban infiltrators dressed as soldiers, by insurgents who have got themselves recruited and Afghan soldiers angry about their treatment because of personal insults or cultural differences.
US commanders had already assigned soldiers to guard their comrades as they slept, ate or interacted with Afghan forces because of the increasing number of "insider killings". American troops were also ordered to carry loaded weapons at all times, even inside their own bases.
Nato attacks on Afghan civilians have added to the strain. In the latest, an air strike killed eight women and girls collecting firewood.
The loss of trust in the force the US is relying on to prevent the Taliban taking control of Afghanistan again, compounds other concerns about Washington's strategy. The additional 33,000 soldiers Barack Obama despatched two years ago as part of the "surge" are expected to complete their withdrawal this week. The remaining 68,000 US troops are supposed to gradually shift responsibility to Afghan forces which, under the American strategy, are to take the lead in combat as early as next year.
But despite gains on the battlefield, questions persist about whether the Afghan forces will have the ability and will to keep an undefeated Taliban at bay once Nato forces have left.