The U.S.-led coalition in Iraq and Syria appears to have used white phosphorous-loaded munitions on at least two occasions in densely populated areas of Mosul and in the Islamic State’s de facto capital of Raqqa, according to videos posted online and human rights groups.

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 phosphorous 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 phosphorous. 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 phosphorous 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.

U.S. Army Col. Ryan Dillon, a spokesman for the U.S.-led coalition fighting in Iraq and Syria, would not confirm the use of the munition but said in an email that the U.S. military uses it in “accordance with the law of armed conflict” and that white phosphorus rounds are “used for screening, obscuring, and marking in a way that fully considers the possible incidental effects on civilians and civilian structures.”

“The coalition takes all reasonable precautions to minimize the risk of incidental injury to non-combatants and damage to civilian structures,” he said.

The Pentagon posted photographs of Marine M777 howitzers in Syria – deployed to support the Raqqa operation – with a pallet of white phosphorous munitions in May. The image was taken in March, and while the unit in the photograph probably has returned to the United States, its replacement is likely using similar munitions.

Mary Wareham, the advocacy director at Human Rights Watch’s arms division, said in email that the group is still trying to determine the veracity of the videos, but the munitions look similar to the ones used Saturday in the Iraqi city of Mosul.