The North Atlantic Treaty Organisation (Nato) Secretary-General Jens Stoltenberg warned on Tuesday of a “high price” if Western allies pull troops out of Afghanistan too quickly, saying it could allow Islamic State militants to regroup.

US President Donald Trump has made ending the 19-year-old American engagement in Afghanistan a priority by Christmas, but while many NATO allies also want to leave the country, many feel it could be reckless given militant violence plaguing Afghanistan.

“The price for leaving too soon or in an uncoordinated way could be very high,” Stoltenberg said in a statement.

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

Trump fired his defence secretary, Mark Esper, and appointed other top Pentagon officials last week, in part because of frustration that not enough was being done to meet his Christmas withdrawal target.

But Reuters reported on Tuesday that Trump's new Pentagon team has not yet signalled an imminent withdrawal of all US troops from Afghanistan, raising expectations among US officials and allies that Trump might settle for a partial reduction before leaving office.

The United States has around 4,500 troops currently in Afghanistan. Including the United States, there are under 12,000 NATO troops overall in Afghanistan, Stoltenberg said in his statement.

Acting Defence Secretary Christopher Miller, Esper's replacement, spoke to Stoltenberg by phone on Nov. 13 and discussed coordination in Afghanistan.