Any student of history or even prolific reader who spends any time out on the web these days runs into endless posts by either the ignorant or malicious making nonsense claims or denials about various unsavory US actions or incredible propaganda.
Here’s one such post by someone with the moniker falconflight, both denying and justifying the use of white phosphorus by the US military at the same time.
I have no proof either way from the US Gov’t and certainly not from the Syrian/Russian/Iranian side. I don’t know that it is BS. BTW, phosphorous use is NOT illegal, except in explicitly outlined limitations, such as high concentration of civilian populations.