(washingtonpost) -- The often-controversial munitions are common in western militaries and are used primarily to create smoke screens, though they can also be dropped as an incendiary weapon. When a white phosphorus shell explodes, the chemical inside reacts with the air, creating a thick white cloud. When it comes in contact with flesh, it can maim and kill by burning to the bone.

While international humanitarian law stipulates that civilians must be protected from all military operations, it also says that countries must take even more care when using white phosphorus. Additionally, because of the weapon’s ability to cause grievous and inhumane injuries, rights groups caution against using white phosphorus to kill enemy troops if other weapons are available.

On Thursday, footage posted by the activist group Raqqa is Being Slaughtered Silently showed the signature spread of airburst white phosphorus munitions — probably M825 series 155mm artillery rounds — exploding over eastern Raqqa, the same area where U.S.-backed Syrian fighters made advances earlier this week.