The United Nations Office for the Coordination of Humanitarian Affairs said that 26,000 people had fled areas where Assad terrorists backed by Russian air coverage have been waging a fierce offensive.
Those areas are located east of al-Bab town, which Turkey-backed opposition fighters seized from ISIS on February 23 after several months of fighting.
The UN agency said the nearly 40,000 people displaced from the town since November fled north to areas controlled by other opposition forces.
Many have sought refuge in areas around Manbij, a town controlled by the US-backed –so-called - Syrian Democratic Forces (SDF).
In Manbij, long queues of families were still forming at checkpoints leading to the town on Sunday, AFP reported.
Pick-up trucks full of children and women wearing full black veils were being searched by SDF personnel before being allowed to enter, AFP added.
Turkey launched a military campaign, called Operation Euphrates Shield, inside Syria in August, backing opposition fighters to fight ISIS. Ankara views the Syrian Kurds who lead the SDF as terrorists.
The SDF are pressing an advance towards retaking the group’s de-facto capital of Raqqa, backed by a US-led coalition that has been carrying out air strikes in Syria and Iraq since 2014.