Syrian state media earlier reported that government forces had entered Manbij in a broadcast it said was from the city.
Youssef Hammoud, spokesman for the Turkey-backed National Army, said that while some Syrian government forces had entered the city, the bulk were Kurdish fighters who had deployed from the Syrian government-held city of Aleppo.
"The forces that entered Manbij are said to be regime forces but most of them are members of the PYD," he said, a reference to the main Syrian Kurdish party.
National Army fighters would continue to advance towards Manbij, he added.
