Civilian militia groups that have sided with US troops in the battle against al-Qaeda in Iraq are threatening to withdraw after three fighters were killed in a suspected friendly fire incident.

The men, from the so-called Awakening group, died before dawn on Friday in Jurf al-Sakher, a village 75 miles south of Baghdad. The deaths led to the reported resignation of scores of guards.

One fighter who survived the attack by US helicopters said in broken English that he had yelled: “My Friend. Mister”. But it was to no avail. “They didn't stop, so we ran away,” he added.

The start of the campaign against al-Qaeda by bands of former Sunni insurgents and others is one of the most significant developments in Iraq of the past year.

The US military said that it had attacked a structure in the area after gunfire. It was unable to say whether the incoming shots had been fired by insurgents, had been discharged in error or had come from rogue elements in the local Awakening group.

The episode, which is under investigation, prompted other guards to threaten to return to resistance. It also demonstrated fears among US forces that al-Qaeda supporters have infiltrated these groups, which have been credited with helping to reduce violence by more than 60 per cent.

Abdul Karim-Khalaf, 42, the head of an Awakening unit formed two months ago in al-Multaqa, near the northern oil city of Kirkuk, said: “The Americans must respect the Awakening guards because they asked us to start this project and said we were their allies. Now they are killing us by mistake.

“This kind of mistake could cause people to take up arms again against the occupying forces.”

Rear Admiral Gregory Smith, a US military spokesman, emphasised that US forces never deliberately targeted civilian guards, also known as concerned local citizens.

“In a couple of cases we do know that our forces were receiving fire from a group of individuals on the ground and we returned the fire and later determined that some of those individuals were in fact concerned local citizens,” he said.

The guard from Jurf al-Sakher said that he and his colleagues fired into the air when they heard movement near their checkpoint. “Straight away they started shooting at us. Helicopters came. We kept shouting ... but they did not stop,” he told al-Sharqiya television station in comments broadcast yesterday.

Awakening groups, which were set up in the once restive Anbar province, west of the capital, exist all over Iraq, and now number about 80,000 largely Sunni Arab but also Shia Arab men under the payroll of the US military. The ultimate plan is for these units to join the regular security forces or for members to find other work in the public or private sector.

Times Recommends

• Robert McNamara dies aged 93
• German soldiers earn new ‘Iron Cross’
• Great Barrier Reef 'gone within 20 years'