In videos released by ISIS' Amaq News Agency, what appears to be several white phosphorous bombs are seen exploding over Raqqa.
The videos join a series of images from Mosul released in recent weeks also depicting what would appear to be phosphorous bombs exploding.
The use of phosphorous munitions in populated areas is forbidden under international law. Like napalm, white phosphorous causes agonizing burns and sticks to the skin, which in some cases can lead to death.
In an interview with the New York Times, an American official said that coalition forces fighting ISIS in Iraq and Syria have access to white phosphorous, but do not use it against human targets.
From the videos, it is unclear whether the areas where the weapons were used are indeed populated, as many have fled Raqqa in the last few months. However, the UN estimates that some 160,000 residents remain in the city.
Col. Ryan Dillon, spokesman for the International Coalition, told the Washington Post that the US military was using phosphorus "in accordance with the laws of war."
He noted that phosphorus is used to "mask, obfuscate, and mark targets in a manner that carefully examines possible effects on civilians and civilian structures."
Coalition forces, including the Syrian Democratic Forces and their Kurdish fighters, are currently engaged in a campaign to conquer the city, with gains being reported in the Romaniya neighborhood and the Al-Meshled district.