The top military officer in the United States says more American troops will probably have to be sent to Afghanistan if the war is going to be won.

The chairman of the Joint Chiefs of Staff, Admiral Mike Mullen, says he does not know if the commander in Afghanistan is going to ask for more troops or how many.

But he says it is clear to him that more forces may be needed.

"A properly resourced counter-insurgency probably means more forces, and without question, more time and more commitment to the protection of the Afghan people and to the development of good governance," he said.

Admiral Mullen has also told the Senate Armed Services Committee that the US is facing a race against the clock to reverse its fortunes in Afghanistan.

"I have a sense of urgency about this. I worry a great deal that the clock is moving very rapidly."

With a new opinion poll showing 58 per cent of Americans are opposed to the war, Admiral Mullen has stressed the importance of the conflict.

"I'd say it is the epicentre of terrorism right now," he said.