American forces accidentally killed nine Iraqi civilians, including a child, while hunting al-Qaeda fighters south of Baghdad, the US military admitted today.

Three other civilians, including two children, were wounded in one of the worst incidents of its kind in several months.

The US military released few other details, but Iraqi police said US helicopters were called in after an American convoy was attacked near the town of Iskandariyah, an insurgent stronghold 30 miles south of Baghdad in the so-called Triangle of Death.

The aircraft apparently fired by mistake on a checkpoint manned by Concerned Local Citizens – Sunnis, some former insurgents, who have turned against al-Qaeda and allied themselves with the Americans. These so-called Awakening Groups have helped greatly to reduce the level of violence in Iraq. The police said women were amongst the victims.

"This incident is under investigation. We offer our condolences to the families of those who were killed in this incident, and we mourn the loss of innocent life," the US military said in a statement.

Its statement was issued as the Iraqi government announced that it would begin negotiations with US officials later this month on the terms of a long-term bilateral relationship between Washington and Baghdad that will focus, amongst other things, on the role of US troops.

The Muslim Scholars' Association, a group of hardline Sunni clerics with suspected links to al-Qaeda and other terrorist groups, claimed the death toll was as high as 20, with 17 members of a single family killed. It said the area where the deaths occurred was a "well-known bastion of those who reject the occupation" and condemned what it described as "these crimes committed by the occupier which continue day after day".

Even the US figures would make the civilian death toll the highest since last October, when a US air strike killed around a dozen Iraqis, including women and children, near Samarra, north of Baghdad.

In November a leader of one of the 'Awakening Councils' of Concerned Local Citizens claimed US soldiers mistakenly killed dozens of his fighters during a protracted gun battle north of Baghdad. The US military admitted killing 25 men, but insisted they were insurgents.

The US military claims insurgents often use civilians as shields.

Times Recommends

• Should Knox’s trial even have reached the courtroom?
• 'Was it real?' The US bailout nightmare
• Iranian opposition plays the long game