Al -Qa'ida could re-emerge in Afghanistan to threaten the West after Nato troops leave the country in two years' time, alliance officials conceded yesterday.
A US general also warned that Nato would face combat with the Taliban until the last day of the mission in Afghanistan.
The downbeat assessments of Afghanistan and its prospects emerged even as Nato leaders met at a summit in Chicago and outlined plans to withdraw their forces by the end of 2014.
US President Barack Obama declared that the Afghan war was "effectively over", while David Cameron said the handover plan was "on track and on target".
But despite their optimistic appraisal, Nato officials conceded the possibility of al-Qa'ida and other groups returning after the West's withdrawal. "It is unrealistic to assume that Afghanistan is going to be completely secure and there is no possibility of a terrorist threat re-emerging," said a senior British official.
"But our achievements of the last decade mean that we will withdraw from a country where the threat is much reduced and where Afghan forces are now much more able to respond to that threat."
The potential return of al-Qa'ida has prompted British generals to draw up plans to keep up to 200 special forces soldiers in Afghanistan for counter-terrorism missions after 2014. Mr Cameron had yet to approve those plans, officials said. Nato forces entered Afghanistan in 2001 after al-Qa'ida's September 11 attacks on the US.
The alliance is preparing to end its mission there, arguing that the Afghan government will soon be able to secure the country on its own.
The Chicago summit announced a "milestone" will be reached in the middle of next year, when all Afghan provinces will be in a "transition" process that will see Afghan forces in charge of security operations. Mr Cameron is expected to use that moment to withdraw several thousand of Britain's 9,500 troops.
He described 2013 as "an inflection point" in the campaign and insisted that the Nato mission had been a success. "Our troops can leave with their heads held high," he said.
The US has also started reducing its numbers. After 2014, "the Afghan war is over, but our commitment to friendship and partnership with Afghanistan continues", Mr Obama said. (© Daily Telegraph, London)