Washington, Dec 21 (IANS): The Donald Trump administration has ordered the US military to start withdrawing roughly 7,000 troops from Afghanistan, a move that is likely to plunge the war-torn country further into chaos.

President Trump made the decision to pull out the troops -- about half the number the US has in Afghanistan now -- at the same time he decided to pull American forces out of Syria, a defence official was cited as saying by the New York Times on Thursday.

The withdrawal of the troops could take months and the order marked a significant departure from Trump's August 2017 decision to slightly increase the number of US troops in Afghanistan and keep them in place with conditions on the ground dictating withdrawal, reports say.

The Afghanistan directive also came as the US was attempting to negotiate a peace deal with the Taliban to end the 17-year-old war. According to the Times, the abrupt troop withdrawal decision stunned Afghan officials, who said they had not been briefed on the plans.

The President's orders on Syria and Afghanistan led to the resignation on Thursday of his Pentagon chief James Mattis, who disagreed with him on a hasty withdrawal from both countries.

Trump's decision was also met with opposition by some of his senior Cabinet officials, including outgoing White House Chief of Staff John Kelly and White House National Security Adviser John Bolton.

The Times said that the troop withdrawals and the resignation of Mattis leaves a murky picture for what is next in the US' longest war and they come as Afghanistan continues to witness spasms of violence in its capital Kabul and other important areas.

The US has about 14,000 troops in Afghanistan, most of which are present as part of a larger NATO-led mission to train, advise and assist Afghan forces. Any withdrawal would be complicated by the fact that the US is part of NATO's Resolute Support mission.

Senior Afghan officials and Western diplomats in Kabul woke up to the shock of the news on Friday. Several of them said that they had received no indication in recent days that the Americans would pull troops out.

Republican Senator Lindsey Graham tweeted that any withdrawal of troops from Afghanistan would be a "high risk strategy" which could reverse US progress in the region and pave the way towards a "second 9/11".

Trump had repeatedly before his election campaign in 2016, publicly advocated leaving Afghanistan -- where US forces have been since 2001.

However, in 2017 he indicated he would keep boots on the ground in the country indefinitely to prevent the country's collapse amid a Taliban resurgence.

American-led combat operations against the group officially ended in 2014, but in the years since the group's power and reach has soared.

In September 2017, Trump announced the US would send 3,000 extra troops to the country amid a shift of strategy.