Col Sean Ryan told Agence France-Presse in a statement: "Combined Joint Task Force - Operation Inherent Resolve has begun the process of our deliberate withdrawal from Syria". Turkish and United States troops are operating in Syria without the consent of Damascus, but U.S. forces will leave the country once Daesh is defeated and Kurdish fighters are assured protection, USA national security adviser John Bolton announced on Sunday.
"On Thursday, some American forces withdrew from the Rmeilan military base in Hassakeh province", said Rami Abdel Rahman, the head of the Britain-based war monitor, which relies on reports from activists on the ground.
The Syrian Observatory for Human Rights said a convoy of about 10 armoured vehicles and some trucks pulled out from the north-eastern town of Rmeilan on Thursday night and crossed the border into Iraq.
President Donald Trump announced last month the decision to withdraw 2,000 U.S. troops who have deployed to Syria in support of the Kurdish-led Syrian Democratic Forces (SDF) militia, which have spearheaded the fight against the IS group.
Trump's shock announcement on December 19 that he was withdrawing all 2,000 American troops from the conflict-wracked Middle Eastern country has led to concern among allies, including Israel, and prompted the resignation of his then defense chief Jim Mattis.
USA national security adviser John Bolton said Sunday American troops will not leave northeastern Syria until ISIS is defeated and US -backed Kurds were protected. Secretary of State Mike Pompeo, who is on a tour of the region, has also sought to reassure the Kurds that they will be safe after USA troops withdraw from the country.
After initially tweeting that he would bring back USA troops "now", Mr Trump this week said the U.S. would "be leaving at a proper pace" while "continuing to fight IS and doing all else that is prudent and necessary".
Foreign Minister Jean-Yves Le Drian of France, which is part of the USA -led coalition, welcomed what he believed was a slower withdrawal by the U.S after pressure from its allies.More news: Wawrinka comments on Roger Federer, Rafa Nadal, Novak Djokovic's dominance
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The news follows weeks of confusion over Donald Trump's plans for USA withdrawal from the country. 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.
A USA defence official said on Thursday that the military had removed some equipment from Syria. Russia, the Syrian government's most vital ally, would be happy to see the US pull out entirely of the war-torn country, where it has military bases and a significant troop presence.
"President Trump has made the decision to bring our troops home from Syria".
While the US military pullout is now officially underway it has made a slow start.
Foreign Minister Mevlut Cavusoglu went so far on Wednesday as to say that if the USA withdrawal from Syria was delayed, "then we will put this decision into practice", suggesting Turkey could go after the Kurds in short order. The latter will no doubt prove the more hard objective to meet. Turkey views the dominant Syrian Kurdish groups as a national security threat.
Turkey was a rare ally that lauded Trump's momentous decision on Syria, a country where it will now have a freer rein to target US-allied Kurdish fighters who have played a major role in the war against IS but are deemed terrorists by Ankara.
USA troops have been involved in Syria's war since 2014 when the first elite force arrived in the country to advise Kurdish-led fighters who were involved in battles against the Islamic State group.
In their Wednesday meeting, Trump, Shanahan and Dunford were also set to begin formulating a more fine-tuned withdrawal plan, according to several defense and administration officials.