Images released by Islamic State's propaganda arm purport to show people inspecting damage from airstrikes and artillery shelling in Raqqa in May. Credit:AP It was followed by the downing of a drone linked to the regime near al-Tanf, in a key region where the borders of Iraq, Syria and Jordan meet. US troops are based in the area. Russia said at the time that the US action would hamper efforts to find a political solution to the conflict and had violated the sovereignty of Syria, one of Russia's closest Middle East allies. "Lavrov expressed his categorical disagreement with the US strikes on pro-government forces and called on him to take concrete measures to prevent similar incidents in future," the ministry said.

Images and reports from witnesses in the northern Syrian city of Raqqa suggest that the US-led coalition battling IS there has used munitions loaded with white phosphorus, the use of which in populated areas is prohibited under international law. Syrian Democratic Forces fighters near the northern Syrian town of Tabqa. Credit:AP Photographs and video clips posted online showed blinding spots of light spreading outward on Thursday night over what residents said was eastern Raqqa. By day, the images showed low white puffs trailing tentacles of white smoke. Both are typical visual signatures of white phosphorus, which can be loaded into artillery shells. Residents reached by text message reported similar bombardments on Friday. US forces patrolling on the outskirts of the Syrian town of Manbij. Credit:Arab 24/AP

The images were distributed by the IS propaganda arm Amaq, as well as an anti-IS monitoring group called Raqqa Is Being Slaughtered Silently. White phosphorus, along with other incendiaries, has been used by Syrian government forces battling insurgents in Aleppo and elsewhere. It is not illegal under international law for militaries to possess and use white phosphorus, and the US and other Western militaries say they use it mainly to create smokescreens to hide troop movements. But it can also be used as an incendiary weapon and like thermite and napalm, it is proscribed in civilian areas by international law. A US official acknowledged that US forces fighting IS in Iraq and Syria have access to white phosphorus munitions, but he said it was not being used against personnel. The official spoke on the condition of anonymity. The spokesman for the US-led task force that is fighting the militants, Colonel Ryan Dillon, said that as a matter of policy he could not discuss the use of specific munitions. But he added that "in accordance with the law of armed conflict, white phosphorus rounds are used for screening, obscuring and marking in a way that fully considers the possible incidental effects on civilians and civilian structures".

Tens of thousands of civilians are believed to still be in Raqqa, even as many IS leaders have fled south to Mayadeen in Deir al-Zor province. UNICEF, the United Nations Children's Fund, warned that 40,000 children are believed to be trapped in the city. Residents said that most of the IS fighters left in Raqqa were local recruits, along with some foreign fighters, and that the most experienced commanders and fighters have decamped. Abdullah, a Raqqa resident living in Beirut, said his relatives had seen what they believed was white phosphorus being used in the city. He also said that an internet cafe had recently been hit by missiles, killing around 20 people who were trying to reach relatives for possibly the last time after IS threatened to shut down all internet providers. One of those killed in the cafe was an activist sending a report to the British-based Syrian Observatory for Human Rights, according to the group's founder, Rami Abdulrahman. The assault on Raqqa is being carried out by the Syrian Democratic Forces, made up of Syrian Kurdish and Arab militias that are receiving arms, training and air support from the US and its coalition partners.

Residents say they have received contradictory instructions from the coalition's recent leaflet drops, with the latest urging them to shelter in place after earlier warnings to leave the city. Doctors Without Borders issued a statement warning that civilians faced dangers, whether staying or trying to flee. "Parents have to make an impossible decision," said Puk Leenders, an emergency coordinator with the group. "Either they stay in Raqqa, subjecting their children to increased violence and airstrikes, or they take them over the front line, knowing they will need to cross minefields and may be caught in crossfire." Reuters, New York Times