U.S. troops crossed into Iraq on Monday (October 21) as part of a withdrawal from Syria that was ordered by President Donald Trump - a move that opened the way for Turkish troops to launch an offensive against Kurdish fighters in the area.
Some Syrian civilians threw stones and shouted abuse at a convoy as they headed towards the border.
Nearly 1,000 soldiers are leaving the northeast of the country to continue the campaign against Islamic State militants in Iraq.
Though on Monday the Pentagon said it was looking into keeping some U.S. troops there, to ensure that oil fields did not fall into the hand of Islamic State or others.
Last week Turkey agreed to pause its offensive for five days under a deal agreed between Washington and Ankara to allow time for the Kurdish fighters to withdraw from a 'safe zone' Turkey aims to establish near its border with Syria
The truce expires late on Tuesday (October 22).
Turkish President Tayyip Erdogan has warned that Turkey will resume its military assault in Syria when the deadline expires if the Kurdish-led Syrian Democratic Forces have not pulled back from the proposed 'safe zone'.
A witness in the region said Turkish forces had begun establishing two of a dozen planned observation posts in the area - drawing criticism from Iran.
An Iranian foreign ministry spokesman said the issues should be resolved by ''diplomatic means'' and that Syria's integrity ''should be respected.''
Trump's decision to withdraw troops was criticized in Washington and beyond as a betrayal of Kurdish allies who had fought for years alongside U.S. troops against Islamic State.