By Justin Webb
BBC News, Washington
America's top soldier - chairman of the joint chiefs of staff Admiral Mike Mullen - has dismissed claims that Nato's Afghan mission is in difficulty.
He told the BBC it would take time for Afghanistan to be pacified - but that the US and its allies had to remain committed to the fight.
Admiral Mullen acknowledged that some parts of the country were heading in a strongly negative direction.
But he said the complete picture across Afghanistan was mixed.
And he disagreed with a recent claim by respected US military think-tank, the Atlantic Council, that there was a "strategic stalemate" in Afghanistan.
Admiral Mullen said it would take a long time but losing was not an option or a possibility.
America would remain committed and the Nato allies should as well, he said.
The military chief has just come back from Pakistan - where he held what he said were successful talks on the safety of the country's nuclear arsenal in the event of further turmoil caused by Islamic extremists.
He told the BBC he was "comfortable" the weapons were entirely in Pakistani control.