Civilians were killed on Thursday when US-led airstrikes targeted Syria's northern city of Raqqa with white phosphorus, pro-government Sham FM radio reported. 

The "unprecedented" airstrikes targeted the neighborhoods of Mashlab, Haret al-Bado, and Rmaileh, said the report, adding that the number of civilian casualties has not yet been clear but it estimated the deaths at tens.

White phosphorus, which is made from a common allotrope of the chemical element phosphorus, are mainly used in smoke, tracer, illumination, and incendiary munitions. When a white phosphorus shell explodes, it can maim and kill by burning to the bone.

Smoke rises from the al-Mishlab district at Raqqa's southeastern outskirts, Syria June 7, 2017. /Reuters Photo via Xinhua

The use of incendiary weapons against civilians was banned by signatory countries in the 1980 Convention on Certain Conventional Weapons Protocol III.

Activists from Raqqa posted footages purporting to show the US-led attack, showing the skies of Raqqa lit by a rainfall of glowing rockets hitting the city.

Meanwhile, a Washington Post report said that the US-led coalition also appear to have used white phosphorus-loaded munitions in densely populated areas of Mosul, a city long been tortured by the chaotic situation in Iraq.

The Pentagon has admitted to killing roughly 500 civilians in the nearly three-year-old war against the ISIL in Raqqa. However, the UK-based Monitoring groups said that roughly 3,800 civilians have been killed by the US-led coalition.