American forces have started to leave the Rmeilan military base in Hasakeh province, according to an activist group
The US-led military coalition in Syria has begun pulling out troops, a spokesman said on Friday without elaborating on locations or timetables.
"CJTF-OIR has begun the process of our deliberate withdrawal from Syria," spokesman Colonel Sean Ryan told the AFP news agency in a statement, referring to the US-led anti-Islamic State group (IS) force.
"Out of concern for operational security, we will not discuss specific timelines, locations or troops movements," he said.
The Syrian Observatory for Human Rights activist group reported that the coalition had started scaling down its presence at Rmeilan airfield in the northeastern province of Hasakeh.
"On Thursday, some American forces withdrew from the Rmeilan military base in Hasakeh province," Rami Abdel Rahman, the head of the Britain-based organisation, said.
"This is the first such pullout of American forces since the US president's announcement" of a full troop withdrawal from Syria last month, he said.
On 19 December, Trump said he had ordered the withdrawal of all US forces in Syria, which are believed to number around 2,000.
His announcement, which came after a phone call with Turkish President Recep Tayyip Erdogan, was criticised even within his own camp and is already having major repercussions on the nearly eight-year-old conflict.
Concerns have been raised about the safety after the withdrawal of the US-backed Syrian Democratic Forces (SDF), whose main component the Kurdish People's Protection Units (YPG) have been accused of being terrorists by Turkey.
On Thursday, Turkey said it would launch an offensive against the YPG even if the US delayed the withdrawal of its troops.
"If the [pullout] is put off with ridiculous excuses like Turks are massacring Kurds, which do not reflect the reality, we will implement this decision," Turkish Foreign Minister Mevlut Cavusoglu told Turkey's NTV television on Thursday.
"We are determined on the field and at the table... We will decide on its timing and we will not receive permission from anyone."
The warning comes as Middle East Eye revealed that US diplomats and military officials failed to present any specific details to their Turkish counterparts about Washington's plans to withdraw its forces from northern Syria during National Security Adviser John Bolton's visit to Ankara on Tuesday.
The meeting with Bolton, aimed at coordinating the pullout process, was said to be tense.
Erdogan, who has welcomed the pullout plan, accused Bolton of a "grave mistake" by demanding that Ankara provide assurances on the safety of the Kurdish fighters before Washington withdraws its troops.