Clarifying the Trump administration's stance after conflicting accounts from the president himself, his advisors and the Pentagon, O'Brien said US troops would be at around 4,500 in the coming weeks, and further reductions would continue after that.

"In the early part of next year we're going to be down to 2,500 troops," O'Brien told an online conference of the Aspen Security Forum.

He was speaking after Trump said on October 7 he wanted all US troops home from Afghanistan, where they have been fighting for 19 years, "by Christmas" or December 25.

That statement could impact the peace negotiations between the Afghan government and the Taliban insurgents, amid their original understanding that Washington would not fully withdraw until there was a solid deal between the two sides.

Days after Trump's remarks the Defense Department's top general, Joint Chiefs Chair Mark Milley, appeared to contradict him.

Milley said any drawdown past 4,500 was "conditions-based" and depends on progress in peace talks.

He said the Taliban needed to honor its pledge to reduce the level of violence, where there had been little change in recent months. On Thursday US envoy Zalmay Khalilzad said the insurgents had agreed to reduce attacks and casualties.

Asked about Milley's comments, O'Brien said that as Trump's top advisor, he represents the president's view.

"When I'm speaking, I'm speaking for the president," he said.

He said Trump's call for troops to be home by Christmas was more sentimental, that all presidents have called for the same thing in past wars.

"All presidents .... want the troops home by Christmas," he said.

"We're on a path right now that looks like about 4,500 this fall and a smaller number in January-February. But if the conditions permitted, we'd love to get people out earlier," he said.

"And I think that's the desire the president was expressing."