US General Stanley McChrystal described the current situation as "serious" with violence on the increase.

Addressing the International Institute for Strategic Studies in London, he said that in the past international forces had been under resourced, and in some areas under performed, and that they needed to change the way that they operated.

"The situation is serious and I choose that word very, very carefully," he said.

"Neither success nor failure in our endeavour in support of the Afghan people and government can be taken for granted."

Gen McChrystal’s review of operations in Afghanistan - including a request for up to 40,000 additional troops - is being looked at by the US administration of President Barrack Obama.

He said that while success was still possible it would require “patience, resolve and time”.

At the same time, he acknowledged that they did not have an indefinite amount of time to defeat the Taliban insurgency.

“These efforts will not remain winnable indefinitely. Public support will not last indefinitely,” he said.

Success, he said, would depend upon convincing ordinary Afghans that the coalition forces could deliver a better life for them. That in turn meant changing the way the troops operated.

“We must redefine the fight. The objective is the will of the Afghan people. We must protect the Afghan people from all threats - from the enemy, from our own actions,” he said.

“We are going to have to do things dramatically differently, even uncomfortably differently in the way we operate.”

He said the international forces needed a change of “mindset” as they could not hope to succeed without the support of the Afghan people.

“Our operational coalition culture distances us physically and psychologically from the people we need to protect,” he said.

“At the end of the day your security comes from the people. You cannot build enough walls to protect yourself if the people don’t.”

He acknowledged the coalition had made mistakes in the past.

“It took us longer than I wish it had to recognise this is a serious insurgency,” he said.

“We have under-resourced our operations. In some areas we have not performed.”

He said it was now important that the aims of the coalition were in proper alignment with the resources available to commanders on the ground.

“I think that if we align our goals and resources, we will not have a significant problem. The problem will be if we don’t,” he said.