The US-led military coalition has been accused of using white phosphorus on civilians in Syria and Iraq in defiance of international law.
Human Rights Watch said it had received reports that the incendiary weapons had been used by US and coalition forces during the battle against Islamic State.
White phosphorus burns with a bright-white smoke at high temperatures. It can be used in combat for marking or obscuring military targets but constitutes a war crime if dropped on civilians. It will burn down to the bone when in contact with flesh.
The munitions have been used in Mosul and Raqqa, where hundreds of thousands of civilians are trapped.
Two videos published on social media this month appeared to show the use of white phosphorus over