Terror of 'incorruptible' Afghan who turned his gun on British troops

The sun was just beginning to set as the shura meeting with local Afghan elders came to a close at the Kamparack Pul checkpoint in Afghanistan's Helmand Province on Sunday evening. British troops were returning to their vehicles when they heard raised voices followed by a short burst of machine gun fire.

In a matter of seconds three soldiers lay dead while a fourth was wounded. Their attacker, an Afghan man in a police uniform, was also shot but survived and is currently under guard until he can be questioned.

The assault is just the latest so-called "green on blue" incident in which Afghan policemen and soldiers have turned on the men training them. The attacks have become so frighteningly common that they account for a quarter of British fatalities so far this year. Of the 28 servicemen killed since January, seven have been felled by bullets from supposedly friendly Afghan rifles. The Ministry of Defence has yet to name the three soldiers killed on Sunday evening but two are known to have come from the 1st Battalion Welsh Guards while the third is with the Royal Corps of Signals.

The growing frequency of rogue killings threatens to undermine one of the key pillars underpinning Nato's withdrawal plans from Afghanistan – training up a viable army and police force which can ensure the country does not slip back into civil war once foreign troops leave. Defence chiefs yesterday insisted that the latest attack would not have an impact on training or withdrawal plans. "Though deeply tragic, yesterday's incident and attacks like it will not derail the mission or distract us from the task in hand," insisted the Defence minister, Phillip Hammond.

But it will do little to ease tensions between Nato and Afghan forces.

The Welsh Guards have been hit particularly hard by green on blue attacks. They are currently in charge of training Afghan forces, a task that has become increasingly fraught with danger. Of the five fatalities the Welsh Guards have suffered since the start of the year, three have been in green on blue attacks.

The identity of the Afghan attacker has not been released but he is known to have come from the Afghan Civil Order Police (Acop), a specialist unit which is supposed to be less susceptible to Taliban infiltration than regular units. Acop troops go through much more lengthy recruitment tests and have been trained up as a less corruptible and more dependable than the standard Afghan police.

Working out the motives for the attacks is often difficult. It is rare for an attacker to be taken alive. The majority either melt away into the civilian population or are killed at the scene. The successful apprehension of Sunday's perpetrator will allow British forces to potentially gain intelligence over whether this particular attack involved a Taliban trained sleeper agent, or whether it was a lone wolf attack by someone with a grudge.

Under an agreement with the Afghan government, British forces are allowed to question green on blue suspects for three days before passing them into Afghan custody. Witnesses to Sunday's killings reported that an argument was heard before the gun shots rang out suggesting the attack might have been the tragic outcome of a dispute in a country awash with guns and a tradition of settling scores by them.

The Taliban tend to claim every green on blue attack as one of theirs – a tactic that is designed to undermine morale and create mistrust between Nato forces and their Afghan colleagues.

However a number of rogue killings are known to have had no clear insurgent link. One attack in July 2011, for instance, which killed three British soldiers and was claimed by the Taliban was carried out by a Shia Hazara, an ethnic group known to loathe the predominantly Pashtun insurgents fighting Nato forces.

Nonetheless, while Nato is keen to play down the influence Taliban networks may have in penetrating Afghan government forces there is little doubt that green on blue attacks have increased considerably in the past two years. International forces do not publish detailed breakdowns of rogue killings but a classified coalition report obtained by The New York Times earlier this year concluded that such attacks "are clearly not rare or isolated" and reflect "a rapidly growing systemic homicide threat".

The report found that between May 2007 and May 2011 at least 58 Western service members were killed in 26 separate attacks by Afghan soldiers and the police nationwide – 6 per cent of fatalities. This year alone there have been 18 green on blue attacks resulting in the deaths of 26 Nato soldiers.