An incendiary weapon is not a chemical weapon as defined by the Chemical Weapons Convention, because it harms humans through heat rather than the direct effects of poisoning. They include weapons such as napalm or white phosphorus.
Incendiary weapons are banned by the Convention on Certain Conventional Weapons (also known as the Inhumane Weapons Convention), Protocol III. Article I defines an incendiary weapon as follows:

any weapon or munition which is primarily designed to set fire to objects or to cause burn injury to persons through the action of flame, heat, or combination thereof, produced by a chemical reaction of a substance delivered on the target. (a) Incendiary weapons can take the form of, for example, flame throwers, fougasses, shells, rockets, grenades, mines, bombs and other containers of incendiary substances. (b) Incendiary weapons do not include:
(i) Munitions which may have incidental incendiary effects, such as illuminants, tracers, smoke or signalling systems;
(ii) Munitions designed to combine penetration, blast or fragmentation effects with an additional incendiary effect, such as armour-piercing projectiles, fragmentation shells, explosive bombs and similar combined-effects munitions in which the incendiary effect is not specifically designed to cause burn injury to persons, but to be used against military objectives, such as armoured vehicles, aircraft and installations or facilities.

Article II explicitly disallows the use against civilians and civilian objects.
Unfortunately, only 107 States are party to the Protocol. Syria is not one of them.