Most of the more than 30,000 fleeing civilians in Aleppo province were women and children, the Syrian Observatory for Human Rights said.
It said the army had retaken several villages from IS during a vast offensive that was still ongoing on today.
Most of the civilians who fled went to areas around Manbij, under the control of the Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab fighters backed by the United States that is also fighting IS, it said.