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US-led coalition warplanes have allegedly used white phosphorus munitions in the Islamic State group's Syrian bastion of Raqqa, endangering the lives of civilians, local activists have said.

The aircraft reportedly dropped the incendiary substance, which if used in the vicinity of civilian concentrations can be considered a war crime, over the city on Thursday, the activist-run Raqqa Is Being Slaughtered Silently .

"In the name of fighting IS, the US-backed international coalition is dropping white phosphorous all across East and West Raqqa," the media collective .

"White phosphorous is an incendiary weapon which can melt the flesh off a human to the bone," it added.

White phosphorus burns at extremely high temperatures upon exposure to air, posing a deadly risk to civilians.

IS' Amaq Agency also footage of what appeared to be white phosphorus falling over the city.

The US government has not yet commented on the reports.

The US-led coalition against IS has recently upped the intensity of air raids on Raqqa, as US-backed Syrian fighters battle to take back the city.

The Kurdish-Arab alliance known as the Syrian Democratic Forces [SDF] on Friday seized part of a district on the western edge of Raqqa and fought to advance inside the city's east.

The SDF broke into Raqqa city for the first time earlier this week, months after they launched an operation to capture the extremist stronghold.

The Britain-based Syrian Observatory for Human Rights monitor said SDF fighters took parts of the suburb of Jazra just west of Raqqa's city limits on Friday.

The Observatory said 23 civilians were killed on Thursday night in over two dozen US-led coalition strikes on and near Raqqa.

An estimated 300,000 civilians were believed to have been living under IS rule in Raqqa, including 80,000 displaced from other parts of Syria.

But thousands have fled in recent months, and the UN humanitarian office estimates about 160,000 people remain in the city.