US President Donald Trump greets soldiers after speaking to them during a surprise Thanksgiving day visit at Bagram Air Field, in Afghanistan, on Nov 28, 2019. (OLIVIER DOULIERY / AFP)

WASHINGTON/KABUL - The Taliban welcomed on Thursday an announcement by US President Donald Trump on pulling out American troops in Afghanistan by Christmas.

It was "a positive step towards the implementation of (the) Doha agreement," a spokesman for the Islamist group, Mohammad Naeem, said in a statement.

US President Donald Trump said on Twitter Wednesday US troops in Afghanistan should be "home by Christmas," hours after his national security adviser said Washington would reduce its forces in Afghanistan to 2,500 by early next year

The response came a day after Trump said on Twitter: "We should have the small remaining number of our BRAVE Men and Women serving in Afghanistan home by Christmas!"

It was unclear whether Trump was giving an order or verbalizing a long-held aspiration.

According to a landmark deal between the United States and the Taliban in February, foreign forces would leave Afghanistan by May 2021 in exchange for counterterrorism guarantees from the Taliban, which agreed to negotiate a permanent ceasefire and a power-sharing formula with the Afghan government.

Trump and other officials have said the United States will go down to between 4,000 and 5,000 troops in Afghanistan around November.

Beyond that, officials have said that a reduction will depend on conditions in Afghanistan.

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Trump, who is seeking re-election next month, has made walking away from "ridiculous endless wars" the cornerstone of his foreign policy, even though thousands of troops remain in Iraq, Syria and Afghanistan.

Just hours before Trump's tweet, national security adviser Robert O'Brien said the United States had less than 5,000 troops in Afghanistan currently and would go down to 2,500 by early next year.

"Ultimately, the Afghans themselves are going to have to work out an accord, a peace agreement. ... It's going to be slow progress, it’s going to be hard progress, but we think it’s a necessary step – we think Americans need to come home," O'Brien told an event at the University of Nevada, Las Vegas.

According to a landmark deal between the United States and the Taliban in February, foreign forces would leave Afghanistan by May 2021 in exchange for counterterrorism guarantees from the Taliban

The National Security Council and White House did not immediately respond to a request for comment.

The White House's plan for the drawdown will almost certainly be subject to review should Trump lose his bid for a second term in the Nov 3 election.

Trump's comments could further weaken the Afghan government's leverage during negotiations with the Taliban.

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While the talks have been taking place in Qatar's capital, Doha, scores of Afghan soldiers and Taliban fighters have been killed in clashes. Dozens of civilians have also died in recent weeks.

Testifying before a US House of Representatives committee last month, US Special Envoy Zalmay Khalilzad said that once the number of US troops reached 4,500, the administration "would do an evaluation of ties and actions that we have taken and make decisions on that."

About 2,400 US service members have been killed in the Afghan conflict and many thousands more wounded.

Wednesday also marks 19 years since the United States invaded Afghanistan to topple the Taliban rulers who had harbored al-Qaida militants who attacked the United States on Sept 11, 2001.