Video footage released by Amaq, the news agency run by Islamic State in Iraq and the Levant (Isil), appears to show the deadly substance falling on the Syrian capital on two separate occasions.
Raqqa is Being Saughtered Silently, an anti-Isil campaign group, has also released similar images on social media.
The use of white phosphorous in densely populated areas is banned under international law, though military forces are allowed to use it to create smoke screens.
Similar to napalm, white phosphorus causes agonising and potentially deadly chemical burns which have been known to melt skin off the bone.
— الرقة تذبح بصمت (@Raqqa_SL) June 10, 2017
A US official told the New York Times that US forces fighting Isil in Iraq and Syria had access to white phosphorus but were not using it on human targets.
It remains unclear whether the districts allegedly hit by white phosphorous are densely populated. Thousands have fled Raqqa over the past few months, but the UN estimates that 165,000 remain in the city.
Colonel Ryan Dillon, a spokesman for the US-led coalition, told The Washington Post that the US military uses white phosphorus in “accordance with the law of armed conflict.”
He said white phosphorus rounds were “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,” Col Ryan added.
Mary Wareham, the advocacy director at Human Rights Watch’s arms division, said the munitions dropped in Raqqa look similar to those used a week ago in the Iraqi city of Mosul, where the US-led coalition is also battling Isil.
But Iraqi forces said the thick white smoke spotted in Mosul was not white phosphorous, adding that it had been used to provide cover for civilians fleeing Isil.
Most of Mosul has been retaken from ISIL, but the militants still hold some densely populated neighbourhoods in the western part of the city.
In October Amnesty International, responding to claims that white phosphorous was being used in Mosul, warned against the dangers of deploying it in any form.
“If buried by soil or water, white phosphorus wedges can be temporarily extinguished, but they spontaneously reignite if they are uncovered, presenting a serious hazard for unsuspecting civilians who may accidentally uncover them as they walk through the affected area,” it said.