Colonel Sean Ryan, spokesman for the US-led coalition fighting Islamic State (IS), confirmed the process had gotten under way on Friday amid confusion over how quickly troops would begin to leave.
President Donald Trump shocked the world when he announced that servicemen and women were to pull out of Syria just before Christmas, prompting criticism from defence secretary Jim Mattis and his resignation.
The US-backed, Kurdish-led Syrian Democratic Forces (SDF) said the move had “dangerous implications” for stability and British defence minister Tobias Ellwood said he “strongly disagreed” with the American leader.
It appeared to be enough to prompt Mr Trump to order a slowdown on the withdrawal, but the update Colonel Ryan provided would make it seem as though those reports were exaggerated.
The Baghdad-based official did not give further details on how many vehicles or troop units have been withdrawn, adding: “Out of concern for operational security, we will not discuss specific timelines, locations or troops movements.”
Originally it was announced that the 2,000 US troops in Syria would be pulled out over the course of 60 to 100 days.
But US national security adviser John Bolton later insisted the US would not leave until IS was defeated, with Mr Ellwood having warned that the terror group had “morphed into other forms of extremism” and “is very much alive”.
Foreign Secretary Jeremy Hunt also said he disagreed with Mr Trump, and told Sky News that Britain had become resigned to Syrian President Bashar al Assad remaining in power and Russia gaining a new sphere of influence.
Mr Trump has given differing signals over the future of US involvement in Syria in recent weeks, although has been less vocal on his strategy amid a domestic row over funding for his proposed southern border wall.
After initially tweeting that he would bring US troops back “now”, Mr Trump this week said the US would “be leaving at a proper pace” while “continuing to fight IS and doing all else that is prudent and necessary”.
He has previously taken pride in US involvement in Syria, having praised the work of his military after joining France and the UK in airstrikes against the Assad regime almost a year ago.
But during a Christmas visit to an air base in Iraq, the president warned allies that the US was no longer going to act as policeman for the world.
He said: “If they want us to do the fighting, they also have to pay a price – and sometimes that’s also a monetary price – so we’re not the suckers of the world.”
The news that a withdrawal had begun was noted on Thursday by the Syrian Observatory for Human Rights, based in the UK, which monitors activity in the war-town country through a network of activists.
It said a convoy of ten armoured vehicles and some trucks had pulled out of Rmeilan, in Syria’s north west, into Iraq.