The usage of white phosphorus is restricted under international humanitarian law. Although there can be lawful uses, it must never be fired at, or in close proximity to, a populated civilian area or civilian infrastructure, due to the high likelihood that the fires and smoke it causes spread. Such attacks, which fail to distinguish between civilians and civilian objects and fighters and military objectives, are indiscriminate and thus prohibited.

https://www.amnesty.org/en/latest/news/2023/10/lebanon-evidence-of-israels-unlawful-use-of-white-phosphorus-in-southern-lebanon-as-cross-border-hostilities-escalate/