Col. Sean Ryan, spokesman for the coalition fighting the Islamic State group, said the US started "the process of our deliberate withdrawal from Syria".
"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 about ten armored vehicles, in addition to some trucks, pulled out from Syria's northeastern town of Rmeilan into Iraq.
The surprise move comes amid conflicting statements over when a withdrawal would begin and at what pace since President Donald Trump abruptly announced the USA would leave last month. Secretary of State Mike Pompeo has also sought to reassure the Kurds they will be safe after the US completely pulls out.
"We have to wait to see what Mr. Erdogan has to say with Mr. Trump", said Selcen, "because he (Erdogan) himself managed to persuade Mr. Trump that the United States will be leaving Syria". But determining what the US plan for withdrawing from Syria is has become a complicated task for many. 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.
Military planners calculate the Pentagon might have to send hundreds of additional forces into Syria on a variety of missions in order to safely withdraw more than 2,000 USA ground forces there, according to two U.S. officials. 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.
"We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency", Trump tweeted that day, adding that USA troops would leave in the next 30 days. A USA troop pullout leaves the Kurds exposed to Turkish attacks from one side, and Syrian government troops on the other. The latter will no doubt prove the more hard objective to meet.
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.
The U.S. announcement that Mr. Trump's ordered withdrawal is underway might be enough to convince Turkey to hold fire against America's Kurdish battleground allies, for a while, but Erdogan still wants to go after them, and he could make good on his threats - even on a small scale - at any time.
Turkey considers several Kurdish groups - including the People's Protection Units, also known as the YPG - to be terrorist organizations.
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.