Another 26 people were injured, according to the White Helmets, volunteer first responders who have crews at the scene.
The cause of the explosion is not immediately known.
The blast occurred in the city of the centre, near a fuel storage site, according to the White Helmets.
Azaz, 350 kilometres north of Damascus, is in area north of Aleppo under the control of Syrian militias backed by Turkey. It is just a few kilometres from the border with Turkey.
This is a developing story… Updated at 3:57 pm
