The Taleban have grown in strength and influence as the fortunes of the coalition forces have deteriorated in Afghanistan, the overall commander of US forces in the Middle East said last night.

General David Petraeus said that the challenges in Afghanistan were “serious”, that they required a sustained and substantial commitment and that there were “no quick fixes”.

The new head of the British Army, General Sir David Richards, also warned last night that Nato had yet to find the right formula for success in Afghanistan. General Richards also warned that defeat for the international coalition would have an “intoxicating impact” on extremists around the world. Failure for an alliance as powerful as the Nato presence in Afghanistan would convice terrorists that “anything might be possible”.

The two generals were speaking after a soul-searching summer marked by a rising number of casualties. Since 2001 Britain’s death toll has reached 216 and America’s 830. But both men insist that the mission is achievable.

General Richards, in his first public pronouncement on the campaign since becoming Chief of the General Staff, tried to lift the sense of gloom that has hung over the campaign and to reassure the public that the sacrifices being made would be worth it.

Speaking at Chatham House in London, he said: “Many I know are sceptical that we can succeed [but] I am an optimist. Let me remind you that most of you only 18 months ago had written Iraq off — a disaster about to implode. Yet today, most are cautiously optimistic. We knew the ingredients for success but it took us time to discover the correct formula.

“The ingredients for success in Afghanistan are similar but we have not yet confirmed the correct formula for that country.”

He was confident that under the leadership of President Obama and with the work being done by General Stanley McChrystal, the US commander of Nato forces in Kabul — who has completed a reassessment of the military strategy — the right formula would soon be found in Afghanistan.

General Richards, a former commander of Nato’s International Security Assistance Force in Afghanistan, was clearly trying to move away from the confrontational approach adopted by his predecessor. General Sir Richard Dannatt, who retired last month, persistently warned the Government about shortages of equipment and had asked for another 2,000 soldiers to be sent to boost numbers to 11,000. His request was rejected by Gordon Brown.

General Richards made no reference to equipment or lack of troops. He focused on his belief that failure in Afghanistan would be catastrophic for Britain’s reputation. Victory for the Taleban would have a “hugely intoxicating impact on extremists worldwide”, and would alienate millions of Afghans “who have placed their trust in the West only to have trust shattered”. Failure would also spread instability in nuclear-armed Pakistan.

— Two soldiers killed in separate explosions in Afghanistan were named yesterday as Acting Serjeant Stuart McGrath, of 2nd Battalion The Rifles, and Trooper Brett Hall, of 2nd Royal Tank Regiment.

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