Civilians have long borne the brunt of the fighting in Afghanistan, though incidents where they have been deliberately targeted by Nato troops are rare.

The fact that the murders allegedly committed by the "kill squad" came to light precisely because the US military had launched its own investigation – which was picked up by the press not in Afghanistan, but in the US, where the courts martial were to take place – show that the way it self-regulates and disciplines its personnel can work at critical junctures.

Where the coalition finds itself in hot water far more frequently is on the issue of civilian casualties caused accidentally. The UN calculated that Nato was responsible for 440 civilian deaths last year but many Afghans find that hard to believe.

Even US General Stanley McChrystal, when he was still the top Nato commander in Afghanistan, said with some exasperation that "an amazing number of people" had been shot who were not a threat. The perception at any rate is that many deaths go unreported – something that Taliban propagandists assiduously exploit.

The reason for this is simple. After each engagement, Nato troops are expected to submit a battle damage assessment, reporting what injuries they may have caused to insurgents or civilians and their property. At times this is relatively straightforward. But if the incident in question was a running gun-battle over several hours with artillery and aircraft in support, no one is going to spend time deep in Taliban country trying to collect the requisite evidence to form a forensic account of what just happened.

To do so would often be to invite another battle.

The difficulty of determining exactly what went on in the heat of battle is no solace for the civilians trapped in the middle whose suffering in terms of numbers killed has grown worse each year since 2001.

"People tell us that they are caught in the middle of the conflict and they don't know which way to turn," said Reto Stocker, of the International Committee of the Red Cross. "It is an untenable situation. Civilians must be protected from harm as much as possible, not become victims of the fighting."

Another pitfall for Nato when it comes to properly accounting for the civilian deaths its troops may have caused is the role of its special forces, whose very secrecy – essential to many of their missions – undermines the transparency necessary for a proper investigation.

Last year the US military was forced into an embarrassing turnabout after a special forces team gunned down an Afghan police chief, a prosecutor and three unarmed women, infuriating locals and drawing a sharp rebuke from Kabul.

It was only two months later that it finally apologised for the botched raid, but not before attempting to smear the journalist who had investigated it.