General Sir Michael Rose wants local tribal militias formed in Afghanistan
Coalition forces in Afghanistan have "now reached their limit", according to General Sir Michael Rose, former commander of UN forces in Bosnia.
He said Nato forces should consider forming local tribal militias to help stabilise the country.
Gen Rose warns there are not enough combat troops to continue the momentum against the Taliban.
He made his comments when writing for the military think-tank, the Royal United Services Institute.
Gen Rose, who has recently returned from Afghanistan, says while the international community is clearly committed to a victory there, serious operational problems remain which could still undo NATO's mission.
He also points out that there are only 8,000 servicemen and women in a vast swathe of territory that is home to more than a million Afghans.
Gen Rose's words echo other military commanders' recent warnings.
But the government has insisted it will not send more troops to the British base at Helmand, and that shortage of combat troops is seen as a wider problem for the Nato mission, BBC defence correspondent Caroline Wyatt said.
Gen Rose also suggests forming Afghan tribal militias to aid western forces and the Afghan army.
"By winning the support of the Pashtun tribes who live on both sides of the border and by developing a sympathetic understanding of their complex tribal systems, it should be possible to achieve security in the key eastern and southern areas of Afghanistan," he says.
However, he does believe the war is being won - at least on a military level - for now.
"Afghanistan is not Vietnam, it is not even Iraq. The insurgency war that is being fought in Afghanistan today is militarily winnable and it is slowly being won."
But he warns that any security gains will not endure without more soldiers, good governance and swift reconstruction.
"As in all insurgency wars, winning the confidence and consent of the people of Afghanistan will always be more important than winning any particular tactical level military battle against the Taleban," he says.
General Rose also believes the rotation of British troops every six months creates a lack of continuity, suggesting that one year operational tours would be more effective.
The Secretary of State for Defence John Hutton and Foreign Secretary David Miliband are due to give evidence on Afghanistan and Iraq to the defence and foreign affairs committee on Tuesday.