Admiral Mike Mullen, the chairman of the Joint Chiefs of Staff, was speaking after a two-day trip to Afghanistan, much of which was spent visiting troops, shortly before he is due to step down in October.

During a press conference in Kabul, he highlighted a lack of good governance in many parts of Afghanistan.

He also spoke specifically about Afghan institutions involved in the transition of power from international to Afghan troops and officials, which will eventually see all foreign combat forces leave by the end of 2014.

Some countries including the US have already started troop withdrawals as part of the transition process.

"I think it's fair to say that in the main, Afghan government officials must work on becoming more responsive to the needs and the aspirations of their people," Mullen said.

"We know that some agencies and institutions vital to transition are infiltrated and subverted by criminal patronage networks."

He added: "We must end impunity for criminals who are subverting the state and victimising the Afghan people."

As well as the security handover to the Afghan police and army, the transition process also includes a wide range of local and national government bodies taking on new responsibilities from foreign officials.

Mullen acknowledged that US "inattention" had contributed to the problem.

The US government has spent $51.8 billion on aid to Afghanistan since 2002, much of which goes through contractors.

Experts say corruption is an endemic problem among many officials in Afghanistan and that the government and foreign powers must do more to combat it.

The length of tours for key units could be extended for the drawdown period leading to the end of combat operations in 2014, Task Force Helmand commander Brigadier Ed Davis told The Independent newspaper.

The "spine" of such brigades could be deployed for a year, probably including specialist troops involved in intelligence and those mentoring Afghan forces, rather than front-line soldiers, he said.

"The constant churn of people with whom you have really strong relationships is hard, so I think you need to reduce that by having people in theatre for longer," Davis said.

"I suspect over time we'll see these changes and a larger percentage of people doing longer tours... We are looking at nine to 12 months.

"When you are mentoring, when you are not in charge, you are not leading, you're very much leading from behind.

"Because the 'knowledge depth' is so great, it makes sense to have people here a bit longer."

Britain has around 9,500 troops in Afghanistan. They are based in the central belt of the southern Helmand province, where they are battling Taliban insurgents and training up local forces.

Prime Minister David Cameron wants all British troops out of a combat role by 2015.

Michael O'Neill, the head of Britain's Provincial Reconstruction Team in Helmand, backed Davis's proposals.

"We are at a critical time and it is extremely important that we do have this continuity," he said.

"Most of the people at the PRT are doing more than six months, up to a year, some have extended to 18 months.

"We find this is very helpful in building up relationships with Afghan partners and also in seeing through projects."

Last month Britain announced it would withdraw 500 troops from Afghanistan by the end of next year, but would keep soldiers there in a training role until at least 2023.

A Ministry of Defence spokesman said: "The MoD is always examining how best to generate and sustain our armed forces, including by looking at the length of tours and the intervals between them.

"However the vast majority of personnel serve six-month tours and there are no current plans to change this."