While the use of WP isn’t prohibited under international law, its use in populated areas is forbidden. WP munitions are primarily used to create smokescreens. However when used as an incendiary weapon, WP — which ignites on contact with air and burns at nearly 1,500 degrees Fahrenheit (815° C) — can horrifically maim and kill by burning flesh straight through to the bone, often causing a slow, agonizing death. Water does not extinguish it.
On Thursday, the human rights monitor group Raqqa Is Being Slaughtered Silently (RBSS) tweeted video footage of what appear to be WP rounds exploding over eastern Raqqa, where U.S.-backed Syrian fighters are advancing deeper into IS-held territory. U.S. Army Col. Ryan Dillon, a spokesman for the anti-IS coalition, would not confirm use of WP in Mosul or Raqqa but said American forces use it in “accordance with the law of armed conflict” 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,” Dillon added.
WP and other incendiary weapons have been used by Syrian government and Russian forces fighting IS and other rebels in Aleppo and elsewhere during the course of the six-year Syrian civil war. U.S. forces used WP during the 2004 battle for Fallujah and elsewhere in the ongoing 15-year war against Islamist terrorism. The U.S. has also supplied key allies with WP, with Israel having been accused of war crimes in 2009 for firing WP rounds over densely populated areas of Gaza, including a United Nations school.
Tens of thousands of besieged civilians remain trapped inside Raqqa, including 40,000 children, according to the United Nations Children’s Fund (UNICEF). Many are caught in the crossfire between advancing U.S.-backed forces and well-entrenched IS holdouts. IS militants are killing civilians who attempt to flee areas it controls, while hundreds of civilians have been killed by U.S.-led air strikes in and around the city.