Make or break time for Afghan forces as Nato prepares to take step back

'We will not let them fail,' says British commander, before local troops take lead role in fighting Taliban insurgency

British commanders have warned that the war against the Taliban is entering its most critical phase as Afghanistan's security forces prepare to fight the insurgency on their own for the first time without Nato troops alongside them on the frontline.

President Hamid Karzai is expected to announce that the Afghan army and police will shortly take the lead in combat operations across the whole of Afghanistan, and senior officers interviewed by the Guardian said the next six months – known as the "fighting season" – would show if the bold strategy had paid off.

In Helmand, where British forces have been based since 2006, commanders believe the Afghan National Security Forces (ANSF) will be able to cope.

But they also insisted that UK combat troops would remain on standby until the end of 2014 to help "in extremis" – and that the ANSF had to try to reduce the number of casualties it has sustained.

Brigadier Bob Bruce, the commander of Task Force Helmand, denied the approach was a gamble, and said it was the right time for UK forces to step back to allow the Afghans to gain confidence before Nato combat troops withdraw next year.

"This is their problem. This is their insurgency. We know for a fact there is no military solution to the insurgency; there is no way the military is going to win a counter-insurgency [war] because it is essentially a political issue. It is a battle of offers: the offer the government makes to the people and the offer the insurgents make to the people."

Bruce admitted the campaign was "at a very challenging stage".

"It is a period of some uncertainties but we reduce the risk by retaining combat capability right to the end, to the end of 2014. We will have the capability to do so, at a reducing scale. We are here to support them if they really struggle."

"I am not interested in gambles. People's lives are at stake. This is a plan at the end of a long campaign. It is a plan that has some risk, but that has been carefully mitigated. I know they are good enough. They are genuinely very capable now."

Bruce said the ANSF was a new and developing force and it needed to be weaned off the support of Nato's International Security and Assistance Force (Isaf). In recent weeks, the Afghans had asked for help on low-level operations but the UK had refused.

"They are finding their feet and they are doing this in contact with a pretty ruthless and determined enemy. They have had a pretty hard fight, as have we. Confidence comes when you have overcome a challenge and this summer will be their biggest challenge yet."

He added: "We will not let them fail. When they really need us, we will intervene."

Lieutenant General Nick Carter, the most senior British officer in Afghanistan, and deputy commander of all Nato forces, said people should not be surprised if UK troops were called into action between now and the end of next year, when Nato formally ends 13 years of operations.

"We want the Afghans to manage this on their own but we do still need to be prepared to support them in the event of this fighting season becoming very intensive.

And that is why it is important that we still have combat power available.

"People should not be surprised if there is a setback that needs to be dealt with, because Afghanistan is still a very difficult place and there is a good deal more work to be done."

Carter admitted the ANSF were taking too many casualties – 1,100 deaths in six months last year, a rate he described as "unsustainable".

He said the high numbers were "indicative of the challenges" still facing the Afghans, and warned it would damage confidence unless it could be brought down.

"Most of their casualties are caused by IEDs [improvised explosive devices]. Their counter-IED capability is developing and they find more IEDs than we do, but they need greater capacity.

"The top of the Afghan army and Afghan MoD [defence ministry] need to recognise they have to drive a culture down through the army, to get leaders to acknowledge that the casualty rates are unsustainable And they have to do something about it," he said.

Carter also took issue with those who used a phrase coined by the Americans, "Afghan good enough", as the benchmark for success. It was first coined by the Americans and has been adopted by politicians and thinkers on both sides of the Atlantic.

"I have never liked that phrase because it is patronising," he said.

"My sense is that Afghanistan has reached a point of progression. Progress now needs to be maintained over the next 10–20 years and we need to help stimulate this mood and this momentum.

"As you travel in that direction you will obviously have speed bumps, and we are not going to solve many of the problems overnight but provided you get some development in civil society terms that becomes irreversible, and I think we are not far away from that point. You can park thoughts like being 'good enough', and you can see Afghanistan being pointed in a direction that is progressive and meets people's expectations."

Nic Hailey, acting UK ambassador in Kabul, said it was "inevitable" that parts of Afghanistan would not be under ANSF control by the end of 2014. He warned that the Taliban were divided internally about whether to enter the political process before next year's presidential elections.