A hasty pullout of US and allied troops from Afghanistan could see the country become a "platform for international terrorists" again, NATO chief Jens Stoltenberg warned Tuesday.

His blunt message came shortly before the Pentagon announced that the US will slash troop levels in both Afghanistan, where they support a NATO mission, and Iraq to their lowest levels in nearly 20 years of war.

"The price for leaving too soon or in an uncoordinated way could be very high," Stoltenberg said.

"Afghanistan risks becoming once again a platform for international terrorists to plan and organise attacks on our homelands. And ISIS (the Islamic State group) could rebuild in Afghanistan the terror caliphate it lost in Syria and Iraq," he said.

The warning was unprecedented criticism by Stoltenberg of a Trump decision. Up to now, the NATO secretary-general has been careful to not publicly contest statements by Trump relating to the Alliance.

But after his defeat in US elections this month by Democrat Joe Biden, Trump -- who remains in power until January 20 next year -- fired his defence secretary Mark Esper, replacing him with Christopher Miller, who said last weekend of American troops in conflicts abroad: "It's time to come home."

In Washington on Tuesday, Acting Defense Secretary Miller said around 2,000 troops would be pulled from Afghanistan by January 15 and 500 more would come back from Iraq, leaving 2,500 in each country.

Stoltenberg in his statement repeated what he said in the past, that NATO troop levels in Afghanistan remained under continual review and that, "when the time is right, we should leave together in a coordinated and orderly way". He said there were currently fewer than 12,000 NATO soldiers in Afghanistan, "and more than half of these are non-US forces".