His comments come amid preparations for a major conference in London - involving more than 60 countries - on the conflict in Afghanistan.

'Way forward'

McChrystal said the arrival of an extra 30,000 US troops this year should deliver "very demonstrably positive" progress.

"It's not my job to extend olive branches, but it is my job to help set conditions where people in the right positions can have options on the way forward," he said.

The additional troops, announced by Barack Obama, the US president, in December, will add to the 70,000 US troops already there.

More than 113,000 international soldiers are fighting the Taliban under US and Nato command in a conflict which started with the US-led invasion of 2001.

Thursday's conference in London is expected to conclude that international forces will face up to five more years battling the Taliban, according to a communique obtained by a British newspaper.

'Physical security'

The Times reported that Afghan forces will take the lead in "the majority of operations in the insecure areas of Afghanistan within three years".

It said the forces will be given up to half a decade for "physical security".

The British defence ministry, which has the second biggest contingent of troops in Afghanistan after the US, said it did not comment on leaked documents.

But it said a decision on pulling out troops would be based on "conditions, not arbitrary timelines".