Vetting procedures for Afghan security personnel in Helmand are being tightened after the killing of two members of the British forces by policemen, The Independent has learnt.

The action follow concerns that rogue elements are slipping through the current safety system to carry out murderous attacks and sow distrust among allies at a key time in the conflict.

Colonel Abdul Ghani Alham, the Helmand provincial police chief, has personally ordered the measures following the latest “green on blue” shooting at a patrol base. Around 50 members of Western forces have died so far from such assaults – raising fears of an enemy within gaining ground as international troops embark on their timetable of withdrawal.

Brigadier Doug Chalmers, the commander of Task Force Helmand, stressed that senior Afghan officers are appalled by what has happened.

In his first interview since arriving in Helmand, Brigadier Chalmers said: “Colonel Alham is as hurt, if not more hurt, than I am.

“I am utterly gutted. The system looks dented and he wants to tighten things up, to ensure it doesn’t happen again. He hasn’t jumped to conclusions, but he wants the investigation done quickly and he is looking at those who vouched for these policemen and he will put in place whatever needs to be put in place.”

The bodies of Lance Corporal Lee Thomas Davies, of 1st Battalion Welsh Guards and RAF airman Corporal Brent John McCarthy returned home yesterday after a farewell ceremony at the British base at Camp Bastion. The two men were gunned down at Patrol Base Attal, in Lashkar Gah district, last weekend.

One of the killers, Sarhad Mohammed, was shot dead, the other, Sardar Ali, escaped. Both the men were 20 years-old, and both had been in the police force for a year after passing through a system put in place after previous incidents of UK soldiers murdered by Afghan colleagues. No motive has emerged so far for the shooting.

The ending of the West's mission in 2014 is predicated on security being taken over by the forces of President Hamid Karzai. This would mean British and other coalition troops working in close proximity to Afghans in the intervening period. Even after the combat role is finished, a sizeable number of foreign forces, including those from the UK, are expected to serve as trainers. This, commanders acknowledge, has raised concerns about an enemy within.

The sense of shock has been heightened because it comes at a time when training and reform appeared to have resulted in significant improvements in the performance of the police and a drop in complaints of corruption and abuse which has dogged the force in the past.

Lieutenant Colonel Dino Bossi, commanding officer of 1st Battalion Welsh Guards, and head of the current training team, said: “There is no doubt that they came to kill, there was no provocation, no arguments; the two men opened fire. Obviously what happened was very distressing, very sad.

“These kind of deaths are very hard to take. But we know we are in a war and we know that there would be losses.”

Brigadier Chalmers said: “There was something at play that day and we are still trying to work out what that was. There are a whole raft of theories, we need to find out who directed it, the reason behind it. Individuals may have tensions, we try and defuse these things, but here guns get involved far too quickly.”

Brigadier Chalmers was at pains to point out that the killings had not undermined co-operation between British and Afghan forces: “They were back out there next day at a shura [public meeting] without hesitation. The relationship is far stronger than when I was here last, there is a sense of condolence and a sense of trust I wouldn’t have seen two years ago.”

Lt Col Bossi had spent a long time with his men discussing what had happened. It has been, he reflected “a humbling experience They are deeply saddened, of course, but no one said they wanted to give up, giving up is not what they do. I am immensely proud of them.”

He added: “What happened was terrible. But I would like to stress that the Afghans themselves are devastated that their own could have done something like this. We shall continue to work together, we need to continue trusting each other.

“What happens if there are more attacks? We’ll have to wait and see.”

The Afghans point out that what does not feature much in the international media is that the enemy has also sought to infiltrate their ranks and they have suffered “green on green” attacks in far higher numbers.

Patrolling the streets of Lashkar Gah yesterday, Naimtullah Fouaz Shafiq, a police lieutenant from Ghorak in Kandahar, said: “The dushman [insurgents] will try to kill those trying to stop them taking over our country, so they kill the foreign soldiers. But we are losing men as well to this type of attacks, we are losing members of our families because we work for the police. The dushman know that if they can divide us then their job will be easier. So we must not be divided, despite these cowardly attacks.”

His colleague Rahimuddin, a policeman of six years who did not want his full name used, added: “We are very sorry for the British who died, we are sorry that they died so far from their homes.

“I think [the] relationship between us is good. It has not been so good sometimes in the past.

“There were some foreign soldiers who were rude to our men, they made jokes, treated them as inferiors. But I have not seen such rude behavior recently so I do not think that was a reason behind this shooting. We need to catch these killers. But this is Afghanistan, and we are going through hard times. We all want the violence to stop, but when that will be, who knows?”

Lt Shafiq added: “It is true that there are lots of checks now before you can join [the police]. In most cases this works, but there are ways people can sometimes get around some of that. Some people have been caught doing this. Even with new rules the dushman will try to get in so we have to be careful.”