The US will slash troop levels in Afghanistan and Iraq to their lowest levels in nearly 20 years of war after President Donald Trump pledged to end conflicts abroad, the Pentagon announced Tuesday.

Acting Defense Secretary Chris 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.

The moves reflect Trump's policy "to bring the wars in Afghanistan and Iraq to a successful and responsible conclusion and to bring our brave service members home," Miller said.

Miller said the US had met its goals, set in 2001 after the Al Qaeda attacks on the United States, to defeat Islamist extremists and to help "local partners and allies to take the lead in the fight."

"With the blessings of providence in the coming year, we will finish this generational war and bring our men and women home," he said.

"We will protect our children from the heavy burden and toll of perpetual war, and we will honor the sacrifices made in the services of peace and stability in Afghanistan, Iraq and around the world."

The announcement came 10 days after Trump fired defense secretary Mark Esper, who had insisted on the need to keep 4,500 troops in Afghanistan to support the Kabul government while it negotiates a peace deal with the Taliban insurgents.

US troops had already been cut by nearly two-thirds from about 13,000 this year, following the February 29 peace deal between the United States and the Taliban.

'Humiliating departure' 

The two sides agreed that the Taliban would then negotiate a peace pact with the Afghan government, and that US troops would be gone by May 2021.

But until Esper's replacement with Miller, Pentagon generals had said that the Taliban had not lived up to pledges to reduce violent attacks on government forces, and that further reductions would take pressure off them to negotiate.

The announcement came over the objections of allies and senior US politicians worried that the reductions would leave the Afghan and Iraq governments vulnerable to extremist groups.

"Afghanistan risks becoming once again a platform for international terrorists to plan and organise attacks on our homelands," NATO chief Jens Stoltenberg warned Tuesday.

"And ISIS (the Islamic State group) could rebuild in Afghanistan the terror caliphate it lost in Syria and Iraq," he said.

On Monday US Senate Majority Leader Mitch McConnell warned the Afghan cuts could lead to a debacle like the US withdrawal from South Vietnam and be a propaganda victory for Islamic extremists.

"The consequences of a premature US exit would likely be even worse than president Obama's withdrawal from Iraq back in 2011, which fueled the rise of ISIS and a new round of global terrorism," McConnell said on the Senate floor.

"It would be reminiscent of the humiliating America departure from Saigon in 1975."

But Trump, who lost his bid for reelection on November 3 to Democrat Joe Biden, has been determined to make good on a campaign pledge made in 2016 to bring US troops home and end costly wars abroad.