President Donald Trump's announcement last month that he had made a decision to withdraw 2,000 US troops stunned allies that have joined Washington in the battle against Islamic State in Syria.
"Out of concern for operational security, we will not discuss specific timelines, locations or troops movements".
"On Thursday, some American forces withdrew from the Rmeilan military base in Hasakeh province", said Rami Abdel Rahman, the head of the Britain-based monitoring organisation.
It said a convoy of ten armoured vehicles and some trucks had pulled out of Rmeilan, in Syria's north west, into Iraq.
"This is the first such pullout of American forces since the USA president's announcement" of a full troop withdrawal from Syria last month, he said.
National Security Advisor John Bolton appeared to contradict the White House Sunday when he said the US withdrawal from Syria would not be immediate, but the Pentagon announced Thursday that without receiving any new direction, it's moving forward with the mission.
There are 2,000 American troops in Syria.
Rami Abdel Rahman, the head of the UK-based monitoring organisation, said some USA forces withdrew from the Rmelan military base on Thursday.
IBM creates Q System One to sell quantum capacity
IBM is enabling universal quantum computers to operate outside the research lab for the first time. The integrated quantum computing system works very similar to the traditional computers.
Foreign Minister Jean-Yves Le Drian of France, which is part of the US -led coalition, welcomed what he believed was a slower withdrawal by the U.S after pressure from its allies. He stressed that the fight against ISIS would continue, but he did no repeat the need for the Kurds to be protected, or mention Bolton's previous vow to curb Iran's presence in Syria.
"The execute order for Syria has been signed", a United States military spokesperson told AFP when asked about the withdrawal order, without providing further details. Russia, the Syrian government's most vital ally, would be happy to see the USA pull out entirely of the war-torn country, where it has military bases and a significant troop presence.
The comments were an example of the perspective that many Russian officials hold, which posits that the U.S.is an imperialist power working to maintain global hegemony.
The White House has not yet publicly altered it's most recent criteria, those announced by Bolton a week ago: that ISIS must be defeated and the Kurdish forces' security guaranteed before the American troops all come out.
Ankara's anger over preconditions announced by Bolton before any USA withdrawal, including security guarantees for the Kurdish militia, may have brought forward the timing of a strike against the YPG. Both had declared that the USA would not leave unless ISIS was entirely defeated and Turkey agreed not to attack Syrian Kurds.
A senior Kurdish politician said the Kurds are aware of the US beginning its withdraw process, describing it as "America's decision".
In the weeks following his announcement, Mr Trump has given differing signals over the future of USA involvement in the country.
Over the past years, the relations between Turkey and the United States have been hampered by Ankara's concerns over USA support for the YPG, viewed by the Turkish authorities as an affiliate of the Kurdistan Workers' Party, listed as a terrorist organisation in the country. If not, the Trump administration would be accused of abandoning an ally to a grim fate.