President Donald Trump is planning to withdraw more than 5,000 of the 14,000 United States troops in Afghanistan, in the latest sign Mr Trump's patience is thinning with America's longest war and overseas military interventions, several US media outlets are reporting.

Key points: The war in Afghanistan began in 2001, after the 9/11 attacks, reportedly to oust the Taliban

The war in Afghanistan began in 2001, after the 9/11 attacks, reportedly to oust the Taliban A withdrawal could allow the resurgent Taliban to further strengthen its hold in Afghanistan

A withdrawal could allow the resurgent Taliban to further strengthen its hold in Afghanistan Critics believe withdrawing US troops could "pave the way toward a second 9/11"

Mr Trump rebuffed top advisers on Wednesday (local time) and decided to pull all US troops out of Syria, a decision that contributed to US Defence Secretary Jim Mattis abruptly quitting on Thursday over significant policy differences with the President.

One official, speaking on the condition of anonymity, said a decision had been made and verbal orders had been given to start planning for the drawdown.

The official said timelines were being discussed, but it could happen in weeks or months.

It is unclear how the US, with less than 9,000 forces in Afghanistan, will be able to fulfil the full set of missions now underway, including training Afghan forces, advising them in the field, and waging an air campaign against the Taliban and other militant groups.

Instead, the US almost certainly would have to curtain its missions, something that could provide an opportunity for a resurgent Taliban to expand their offensives across Afghanistan.

Jim Mattis and other critics have warned a precipitous exit would threaten American national security. ( Reuters/US Army )

Mr Mattis had argued for maintaining a strong US military presence in Afghanistan to bolster diplomatic peace efforts.

He resigned shortly after US officials raised the possibility that Mr Trump would order the drawdown.

The decision on Syria has bewildered US allies and triggered harsh reaction from Mr Trump's fellow Republicans in Congress.

The Pentagon declined to comment on Afghanistan.

Garrett Marquis, a spokesman for the National Security Council, said the White House would not comment "on future strategic developments".

The US went to war in Afghanistan in 2001 in the wake of the September 11, 2001 attacks on the World Trade Centre and the Pentagon, seeking to oust the Taliban militants harbouring Saudi-raised militant Osama bin Laden, who led plans to carry out the attacks.

US officials are currently engaged in talks with the Taliban, who now control a significant amount of territory.

The Taliban insurgency has strengthened its grip over the past three years, with the Government in Kabul controlling just 56 per cent of Afghanistan, down from 72 per cent in 2015, a US government report showed.

'What are we doing there?'

Mr Trump reluctantly increased US troops in Afghanistan last year. ( Reuters: Kevin Lamarque )

Mr Trump has been privately grumbling about US military involvement in Afghanistan, telling an ally as recently as Wednesday words to the effect of, "what are we doing there? We've been there all these years".

The source, who asked to remain unidentified, said it appeared the President "has lost all patience" with the US military presence in Afghanistan.

More than 2,400 US forces have died in the 17-year-old war in Afghanistan, and Pentagon officials have repeatedly warned that a precipitous exit would allow militants to develop new plots on America.

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Republican senator Lindsey Graham, often a vocal Trump ally, warned of possible danger to the US if the drawdown goes through.

"The conditions in Afghanistan — at the present moment — make American troop withdrawals a high-risk strategy," Mr Graham said.

"If we continue on our present course we are setting in motion the loss of all our gains and paving the way toward a second 9/11."

Mr Trump last year approved an increase in US troops, but acknowledged that he did so reluctantly.

Late last month, at least 22 Afghan police were killed in a Taliban ambush in Afghanistan's western province of Farah, adding to the growing casualty toll on Afghan security forces.

Earlier this week, US special envoy Zalmay Khalilzad and Taliban representatives held talks in Abu Dhabi on a deal that would end the war.

Officials from Saudi Arabia and the United Arab Emirates also took part.

The Saudi ambassador to Washington, Khalid bin Salman, tweeted on Thursday that the discussions had been productive and would bring "very positive results by the beginning of next year".

Reuters