It is not illegal under international law for militaries to possess and use white phosphorus, and the United States’ and other Western militaries say they use it mainly to create smoke screens to hide troop movements. But it can also be used as an incendiary weapon, setting very hot fires. And like thermite and napalm, it is proscribed in civilian areas by international law.

A United States official acknowledged that American forces who are fighting the Islamic State, also known as ISIS, in Iraq and Syria have access to white phosphorus munitions, but he said it was not being used against personnel. The official spoke on the condition of anonymity because he was not authorized to discuss the use of the munition.

The spokesman for the American-led task force that is fighting the militants, Col. Ryan Dillon, said that as a matter of policy he could not discuss the use of specific munitions. But he added that “in accordance with the law of armed conflict, 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.”

It has not been determined whether the shells that appeared to contain white phosphorus landed in populated areas, but tens of thousands of civilians are believed to still be in Raqqa, even as many Islamic State leaders have fled south to Mayadeen in Deir al-Zour Province. Unicef, the United Nations Children’s Fund, warned that 40,000 children are believed to be trapped in the city.

Residents said that most of the Islamic State fighters left in Raqqa were local recruits, along with some foreign fighters, and that the most experienced commanders and fighters having decamped.