The chemical weapon is banned under the Geneva Convention, because it is highly toxic and can burn through flesh and bone.

White phosphorus, an incendiary also known as WP, is used by armies to illuminate targets during the night or as a smokescreen during the day.

Shocking images posted to Twitter show the sky allegedly filled with the potentially deadly weapon, as it rains down on the de-facto capital of ISIS, Raqqa.

But while Raqqa may be an ISIS stronghold, activists on the ground claim that civilians were caught up in airstrike, which was described as a ‘war crime’.

Human rights organisations have claimed that Russian airstrikes have killed more civilians than ISIS fighters.

‘To protect the devil (Syrian President) Assad, #Russianairstrikes using phosphorous bombs on #Raqqa #warcrimes,’ wrote a Muslim Iraqi Twitter user who ‘helps innocent civilians’.

Although the photographs haven’t been verified, citizen journalists from the Raqqa is Being Slaughtered Silently network alleged ‘that airstrikes targeted Raqqa today with phosphorus’.

On November 13, witnesses in the opposition-controlled northwest Syrian city of Idlib, told The Times that dozens of civilians suffered ‘horrific injuries’ following two phosphorous attacks.

‘We knew it was phosphorous because the entire sky lit up and when it settled it set everything on fire,’ said Ahmed, an activist based in Idlib.

He added that the two villages that were targeted were full of civilians, and were at least 40 miles from the nearest military posts.

Absorption of the chemical can cause heart, liver and kidney damage and, in severe cases, death.

The Geneva Convention defines ‘incendiary weapons’ as ‘primarily designed to set fire to objects or to cause burn injury to persons through the action of flame, heat, or combination thereof, produced by a chemical reaction of a substance delivered on the target’.

Meanwhile, Russian President Vladimir Putin and U.S. President Barack Obama met today at the climate change summit in Paris, to hold 30 minutes of meetings on the fringe of the conference.

They discussed the ongoing conflicts in Syria and Ukraine, according to a Kremlin spokesman.

Mr Obama stressed the importance of Russia targeting ISIS militants in Syria, as part of its intensive bombing campaign, and not focusing military attacks against rebel groups who oppose Syrian President Bashar al-Assad.

The U.S. President also expressed regret over the recent death of Oleg Peshkov, the Russian pilot whose warplane was shot down a Russian warplane at its border with Syria by a Turkish missile.

The plane downing triggered Moscow's deployment of an advanced missile system to Syria. Obama encouraged de-escalation between Russia and Turkey, the official said.

The meeting came as activists revealed Russian airstrikes have killed more civilians than ISIS fighters in Syria since the start of its devastating bombing campaign two months ago.

The London-based Syrian Observatory for Human Rights estimate that 1,502 people have been killed in Russian airstrikes in Syria.

(dailystar.co.uk)