The withdrawal of American troops from Syria has begun, the US-led coalition against ISIS announced on Friday.
"CJTF-OIR has begun the process of our deliberate withdrawal from Syria," spokesman Colonel Sean Ryan said in a statement, using the acronym for the Combined Joint Task Force - Operation Inherent Resolve.
"Out of concern for operational security, we will not discuss specific timelines, locations or troops movements," he said.
The Syrian Observatory for Human Rights reported that the coalition had started scaling down its presence at Rmeilan airfield in the north-eastern province of Hassakeh.
"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.
"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.
The coalition's announcement came a day after US Secretary of State Mike Pompeo confirmed in an address to regional allies in Cairo that Washington was going ahead with the pullout announced by President Donald Trump on December 19.
While Trump's tweets announcing his decision declared that ISIS had been defeated and suggested the withdrawal would begin immediately, the president later clarified that it would be done gradually after facing resistance from his security officials and the Pentagon.
A US defence official said on Thursday that the military had removed some equipment from Syria.
"I can confirm the movement of equipment from Syria," the official told Agence France-Presse. "For security reasons, I am not going to provide further details at this time."
Trump's shock announcement on December 19 that he was withdrawing all 2,000 American troops from Syria alarmed allies and prompted the resignation of his defence secretary Jim Mattis.
Pompeo is currently on a regional tour to assure Middle East allies of Washington's continued commitment to countering extremists militants in the region and the threat posed by Iran.
In his speech at the American University in Cairo on Thursday, the secretary of state said the US would continue working with coalition partners to defeat ISIS and would continue air strikes against terrorist targets in the region.
"President Trump has made the decision to bring our troops home from Syria. We always do and now is the time, but this isn’t a change of mission. We remain committed to the complete dismantling of ISIS – the ISIS threat – and the ongoing fight against radical Islamism in all of its forms," he said.