General Stanley McChrystal: "In some areas we've under-performed"
Coalition forces in Afghanistan are going to have to adopt a "dramatically different" strategy to ensure success, the top US general there says.
In his first speech since submitting a report calling for more troops, Gen Stanley McChrystal also said the operation had been "under-resourced".
The success of the military operation could not be taken for granted in the face of a growing insurgency, he said.
Meanwhile, a Nato strike in Helmand has killed at least six civilians.
Women and children are believed to be among the dead. Nato has said it will investigate the incident.
It said it had dropped a bomb on a compound in the southern province of Helmand from where insurgents were exchanging fire with security forces.
The protection of civilians is one of the cornerstones of Gen McChrystal's Afghan strategy.
But the White House has said that a review of its Afghan plans may take many weeks.
"We have under-resourced our operations, in some areas we have under-performed, in some areas we have under-co-ordinated and in some ways we have not overcome very intrinsic disadvantages," Gen McChrystal told the International Institute for Strategic Studies (IISS) in London.
'Redefine the fight'
The general warned that forces would have to adopt an entirely new approach when tackling the insurgency.
Paul Reynolds, BBC world affairs correspondent
Gen McChrystal's strategy for success in Afghanistan can be summed up by his phrase: "We don't win by body count - we win when the people decide we win."
Everything he said in his speech was determined by this approach, which is a counter-insurgency strategy, not a counter-terrorism one. The difference is that in the first, the protection of the people is central.
In the second it is the killing of the enemy. The second has been tried and failed, he is suggesting, and it is time to concentrate on the first. The international forces had to do things in a "dramatically" and perhaps "uncomfortably" different way.
The situation was serious and neither success nor failure could be taken for granted. Another key part of his thinking is to reject "bumper-sticker truths" and to remember "humility".
All this will require more troops - perhaps 40,000 more - and the White House is examining the general's proposals.
"We are going to have to do things dramatically differently, even uncomfortably differently in the way we operate."
But the commander said that he thought success was possible.
He reasserted his belief that the focus of military action in Afghanistan must shift towards protecting civilians.
"We must protect the Afghan people from all threats - from the enemy, from our own actions," he said.
Gen McChrystal stressed the importance of the Afghan population's support when tackling the insurgency.
"We don't win by destroying the Taliban, by body count, by the number of successful of military raids. We win when the people decide we win," he said.
Gen McChrystal has requested up to 40,000 more troops on the ground, US officials say.
But the White House said the president would need to assess progress in Afghanistan and warned that the review could last some time.
Gen McChrystal also said it was important to ensure the rapid growth of Afghan security forces.
He also highlighted the need to:
Address how the Afghan government maintains the rule of law
Tackle corruption on the part of officials and warlords
Focus resources on areas where the population was most threatened
Above all, the commander emphasised the need to understand the complexity of Afghanistan's history and current situation.
"You can't assume that what is true in one province is true in another," he said.
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