There have been several accusations of Israel using white phosphorus following the October 7 attack on Israel by Hamas. According to CBS News:

Because of its incendiary effects, the use of phosphorus in war is supposed to be tightly regulated under international law — but it is not banned. Phosphorus is not classed as a chemical weapon under the Chemical Weapons Convention.

When used as a weapon, it can cause fire to rain down on targets, inflicting indiscriminate damage. It is illegal, therefore, for phosphorus to be used near civilians, because international law requires that combatants distinguish between civilian and military elements.