Kim Sengupta: Tensions between UK and Afghan troops on patrol in Helmand

The attack began late in the afternoon on a long, hot day. Raking bursts of machine-gun fire followed by Kalashnikov rounds as British and Afghan troops scrambled into a compound through a narrow doorway.

Within minutes Amarullah Ali was on the wall, standing in full view of the Taliban shooters, unprotected by body armour, a rakishly worn beret instead of a helmet, firing his UGL (underslung grenades launcher) in the general direction of the enemy. The UK troops shouted at him to get down, but the 22-year-old Afghan was not to be deterred.

"This is bloody stupid, he is bringing down fire on himself and us," complained Private Richard Harvey. "Just let them loose," sighed Private Jamie Aitken. "It's an Afghan thing, he just won't listen."

This was Operation Omid Do, in the badlands of Helmand, the first ever Afghan-led military operation in the war and a key part of the West's exit strategy: handing over security to President Hamid Karzai's forces.

The mission to wrest back Yakchal, an insurgent enclave in Nahr-e-Saraj, involved close to a thousand Afghan troops and ended successfully with co-operation between the two forces.

At the compound wall, Private Aitken joined Amarullah Ali, also known as John J, who was declaring "We Afghans are afraid of nothing," and opened fire at a position behind a cemetery to an insurgent sniping point.

A number of other targeted compounds were cleared, some of them heavily laden with IEDs (improvised explosive devices). One of the casualties in the action was an Afghan soldier shot and wounded while guiding a unit of British Army Gurkhas across a field.

Some of the Afghan troops taking part in Omid Do returned to their quarters at PB (Patrol Base) 3 at the end of the operation. Later that night, it is claimed, Talib Hussein, a 22-year-old soldier liked and trusted by the Gurkhas, took advantage of his role on guard duty to load himself up with a variety of weapons to kill three of his British comrades.

It remains unclear what motivated Hussein to carry out the attack. As a Hazara Shia he appears to have been an unlikely recruit for the Sunni Pashtun Taliban. But what happened was undoubtedly a propaganda victory for the insurgency, sowing distrust among the Afghan and British allies which has already had to be repaired once after the killing of five UK soldiers by an Afghan policeman at Nad-e-Ali eight months ago.

"How do we feel about trusting the Afghans? Ask the Gurkhas," said one soldier. "How can you trust them after what happened? We need to keep a loaded weapon with us at all times."

After a pause, he continued. "Look, part of the problem is the language barrier. We don't really have much of a chance to communicate. There aren't enough 'terps' (interpreters) and it's a bit difficult to get to know people unless you get a chance to talk to them."

Another soldier added: "We know it was just one guy, but there was also just one guy at Nad-e-Ali. And between the two of them they killed a lot of people. You can't help wondering just how many nutters you have got out there. But, I have three months still to go and I can't spend every day looking over my shoulder."

Another consequence of the rogue gunman's actions was that they deflected attention away from an operation that had gone far more smoothly than the British troops expected.

Captain John Young, of 1st Battalion The Royal Regiment of Scotland, leading one of the units, said: "With the ANA (Afghan National Army) you tend to take three steps forward and one back. But those two steps are important and we are slowly getting there. I thought they did pretty well today, considering the numbers involved. Maybe it was because it was an Afghan-led operation they felt more motivated. At the end of the day, this is the way ahead."

Many among the Afghan soldiers said that the killings at PB3 had taken away much of the feeling of elation they had felt over Operation Omid Do.

"We were successful, and that is the reason the Taliban and the Pakistanis did what they did at PB 3," said Nasruddin Khan. "Why else would they choose this time to do this? We went into Yakchal and they ran away. Then they kill in this cowardly way. The British who died were warriors, the one who killed them isn't a man."

He insisted that he and his colleagues get on well with most of the British troops although some could "be rude without reason". Nasruddin added: "We are willing to learn but they should understand that we have a hard life here. We have to spend a long, long time in somewhere like Helmand, not just six months."

The posting Afghan soldiers get on leaving the Kabul military academy is a matter of lottery, or bribes. Those sent to the front line of Helmand and Kandahar have to do three years, with the junior ranks getting little chance of any leave for the first year.

"You can imagine the pressure that can build up if someone is exposed to the pressure of life in a PB in a hostile area without much of a break year after year," said a senior British officer. "This is something the Afghan security force needs to look at urgently. It leads to people going on leave and not coming back, but also one has to wonder what kind of mental effect it may have."

Amarullah Ali, from Kunduz, in northern Afghanistan, so keen to make a stand at the wall, agrees that being away from home for such long periods is extremely difficult. "We are here to fight for Afghanistan, but people miss their families, some get sad, some get angry. I do not know what happened at PB3, but sometimes people do stupid things."