US President Barack Obama greets US troops during a surprise visit to Bagram Air Field, north of Kabul, in Afghanistan, on Sunday, prior to the Memorial Day holiday. Photo: AFP

US President Donald Trump said on Wednesday that he wants all American troops to leave Afghanistan by Christmas, speeding up the timeline for ending America's longest war.

"We should have the small remaining number of our BRAVE Men and Women serving in Afghanistan home by Christmas!" Trump wrote on Twitter.

In a February 29 agreement reached in Qatar with the Taliban, the US promised to pull out all its troops by mid-2021 in return for insurgents' promises not to allow Afghanistan to be used by extremists - the original reason for the 2001 US invasion.

After intense US cajoling, the Afghan government and Taliban in September opened peace talks in Doha, although negotiations have quickly deadlocked.

Trump's promise comes one month before US elections in which the president, trailing in the polls, has sought to show that he is making good on his promise to draw a close to "endless wars." 

After 19 years of US military operations his stance enjoys broad support at home including from his Democratic rival Joe Biden, who during his time as vice president had pushed to curtail US involvement in Afghanistan.

Asked in September whether he backed Trump's plans to withdraw US troops from both Afghanistan and Iraq, Biden said: "Yes, I do. As long as he has a plan to figure out how he's going to deal with ISIS," the ultra-violent movement that has been active in both countries.

The US first intervened in Afghanistan following the September 11, 2001 attacks and dislodged the Taliban regime, which had welcomed Al-Qaeda.

AFP