US President Donald Trump will sharply reduce the number of US forces in Afghanistan from 4,500 to 2,500 before he leaves office, the Pentagon has announced, stopping short of the complete withdrawal he threatened to carry out by Christmas.

President Trump's decision to limit himself to a partial withdrawal was first reported yesterday and has triggered warnings from critics who say it will undermine security and hurt fragile peace talks with the Taliban.

Acting Defense Secretary Christopher Miller, who President Trump appointed last week after firing Mark Esper, confirmed the drawdown and also outlined a modest withdrawal of forces in Iraq that will reduce troop levels there from 3,000 to 2,500.

"By January 15, 2021, our forces, their size in Afghanistan, will be 2,500 troops. Our force size in Iraq will also be 2,500 by that same date," Mr Miller said, without taking questions from reporters.

"This is consistent with our established plans and strategic objectives, supported by the American people, and does not equate to a change in US policy or objectives."

Acting Defense Secretary Christopher Miller - "This is consistent with our established plans and strategic objectives"

Moments later, the top Republican in the Senate, Majority Leader Mitch McConnell, warned against any major changes in US defence or foreign policy in the next couple of months - including major troop drawdowns in Afghanistan and Iraq.

US and Afghan officials are warning of troubling levels of violence by Taliban insurgents and persistent Taliban links to al-Qaeda.

It was those ties that triggered US military intervention in 2001 following the al-Qaeda September 11 attacks on the United States. Thousands of American and Allied troops have died in fighting in Afghanistan since then.

Some US military officials had been urging President Trump to keep US troop levels at around 4,500 for now.

The withdrawal stops short of the president's pledge on 7 October, when he said on Twitter that the US should have the remaining number of troops home by Christmas.