They said the blast occurred during peak late night shopping after breaking of the fast during the Muslim month of Ramadan.
"It's timing comes with heavy congestion by shoppers," said Yaseen Shalabi who was near the site of the explosion shopping with his family.
There was no immediate claim of responsibility.
The Arab-populated town run by Syrian rebel groups opposed to Syrian President Bashar al Assad has been relatively quiet since it was hit by a car blast over two years ago.
Main towns in the northwestern border area have in recent years been frequently hit by bombings detonated in crowded civilian areas.