BBC Panorama reporter Ben Anderson spent five weeks with US Marines working to advise Afghan security forces in Helmand province. While he was there, he witnessed corruption and criminality among the Afghan police force.

Most police forces investigate crimes like corruption, kidnapping, drug use, murder and child abuse. But in Sangin - the most violent district in Afghanistan - these are crimes that some of the police commit.

Politicians insist the handover to the Afghan security forces is going smoothly and that they will be able to maintain security as the allied forces withdraw.

On a recent visit to Helmand, UK Defence Secretary Philip Hammond said the "transition is proceeding very well - it is on track.

"The Afghans are developing capabilities faster than we expected and we have every reason to believe that they will be able to maintain security as the Isaf forces draw down," he added.

The outgoing commander of Nato forces, Gen John Allen, is even more ebullient: "Afghan forces are defending Afghan people and enabling the government of this country to serve its citizens. This is victory. This is what winning looks like, and we should not shrink from using these words."

Image caption Ben Anderson has travelled to Helmand province many times since 2007

The reality in Sangin is very different.

I spent five weeks embedded with US Marines, who took over the region from British troops in 2010. I hoped to gain a true sense of what progress has been made in readying the Afghan forces to secure the area.

During my time in Sangin, just two teams of 18 US Marines went out every few days to advise the Afghan army and police across the district.

The remaining American forces have withdrawn to the main forward operating base, which they rarely leave.

Because of the growing risk of deadly insider - or "green on blue" - attacks, the Americans live completely apart from their Afghan counterparts. Whenever the Americans enter the Afghan side of the base, they have their weapons cocked, ready to fire.

When they did go out, what the marines saw was far from encouraging. At one checkpoint, the Afghan police were openly smoking marijuana. Two other police officers, assigned to fill sandbags to fortify a watchtower, were high on something stronger - probably opium or heroin. When one of the police commanders was shot, three weeks after I left, the American medics who saved him found a bag of heroin in his pocket.

Major Bill Steuber is leading the police advisory team, and spends much of his time at headquarters with the police leadership.

There's false imprisonment... they'll wait for the families to come and pay them money to release them Major Bill Steuber, US Marine

He said corruption is rampant, and even compared it to the American television show The Sopranos.

"It's vast," he said, "everything from skimming ammunition off their supplies to skimming fuel off their shipments.

"There's false imprisonment - they'll take people during an engagement, and they'll just wrap everyone up, then they'll wait for the families to come in and pay them money to be able to release them."

He said the police sometimes sell ammunition and weapons in the local bazaar, including rocket-propelled grenades. So weapons paid for by the allied forces could well be ending up in the hands of the Taliban.

In one instance, a patrol base was deemed unsafe to stay in because the Afghan police were selling off the security walls as scrap metal.

Major Steuber said the foreign military working here have to accept the limitations on what they can hope to achieve.

He said that because the Afghan police were unable to sustain themselves, sometimes corruption was the only way they could function.

Panorama: Find out more Panorama: Mission Accomplished? The Secrets of Helmand

BBC One, Monday, 25 February at 20:30 GMT Then available in the UK on the BBC iPlayer

"If we were to go in and shut down all of their schemes, all of their corruption schemes, you would render them completely ineffective," he said.

But there are issues Major Steuber said need to be tackled head-on - including the sexual abuse of young boys by local police commanders.

On every police base I visited in Sangin, there were young boys: some were armed, and some looked like servants. They are known as "chai boys".

Major Steuber says they are often sexually abused.

The problem is widespread. While I was in Sangin, four boys were shot while trying to escape police commanders, three of them fatally. None of the commanders responsible were arrested.

Sangin Deputy Police Chief Qhattab Khan admitted this abuse is taking place, and promised to take action.

He told Major Steuber: "The kids themselves want to stay at the patrol bases and give their bodies at night… There is no humanity. There is no military command".

Mr Khan retired before any action was taken to free the chai boys. To date they have not been released.

"Try doing that day in, day out," said Major Steuber, "working with child molesters, working with people who are robbing people, murdering them. It wears on you after a while."

The Afghan government says it is fighting corruption and that the police and armed forces are ready and willing to take full responsibility for the security of their country.

Ministry of Interior Spokesman Sediq Sediqi, said the Afghan Government would investigate the claims of corruption and abuse, highlighted by Panorama.

But from what I saw, corruption and criminality are widespread among the police in Sangin. This is exactly the kind of behaviour that led many Afghans to welcome the Taliban when they swept to power in 1996. Is this what all the fighting and bloodshed has been for?

Ben Anderson has travelled to Helmand province many times since 2007. He has written a book about his experiences there - No Worse Enemy.

Panorama: Mission Accomplished? Secrets of Helmand, BBC One, Monday, 25 February at 20:30 GMT and then available in the UK on the BBC iPlayer.