Shells Have Recently Been Dropped in Mosul and Raqqa
Though white phosphorus shells are not uncommon in the military, and often used as smokescreens, the high temperature at which it burns, and the toxic chemicals emitted makes them wholly unsuitable for populated areas, and their use in any densely populated area or as an incendiary are widely considered war crimes.
In the cases where images are emerging, they are definitely in densely populated areas, with both Mosul and Raqqa apparently facing the use of white phosphorus. The US would neither confirm nor deny the use, though officials did insist that if they were using them, it was in keeping with their ongoing efforts to limit the number of civilians they killed.
That assurance is less and less valuable, as US airstrikes killing soaring numbers of civilians in Mosul and Raqqa at any rate. Rights groups are warning that the use of the shells in populated areas is an undue risk, and they are seeking more information on the context in which they are being deployed. The increasingly secretive Pentagon is doubtless not going to comply with that.