Amaq, a news agency affiliated to Islamic State, released images over the weekend of what is said were white phosphorus munitions lighting the city of Raqqa that were dropped by the US-led coalition. The agency said 33 people were killed and 25 injured in a series of raids.
Videos posted by Raqqa activists and personal accounts coming out of the city also cited witnesses saying the city was hit for the second consecutive night with incendiary bombs.
Used as an offensive weapon, white phosphorous is self-igniting, burns quickly and produces an immediate smokescreen. Its use is banned in densely populated civilian areas by the Geneva Conventions.
The US-led coalition does not deny using incendiary bombs and says casualties are unavoidable even as it tries to minimize the impact of its military campaign on civilians.
US-backed Syrian forces have advanced into opposite sides of Islamic State's so-called Syrian capital of Raqqa, the forces and a war monitor said on Saturday.
The Syrian Democratic Forces (SDF), a group of Kurdish and Arab militias supported by a US-led coalition, began to attack Raqqa last week after a months-long campaign to cut it off.
The US-led coalition estimates that Raqqa, which Islamic State seized from Syrian rebels in 2014 during their lightning advance in Syria and Iraq, is defended by 3,000 to 4,000 jihadists.
It has been a hub both for Islamic State's military leaders and its bureaucrats, and has been used to plot attacks in countries around the world.
The SDF said it had seized al-Mishlab district in the far east of Raqqa on Friday and al-Sabahia district in the west. The war monitor, the Syrian Observatory for Human Rights, said the SDF had taken Mishlab and more than half of Sabahia.
The forces are now advancing into al-Romania district in northwest Raqqa, the SDF and the Observatory said. Islamic State had turned back an earlier SDF assault on a military base on the north side, the Observatory said.
Islamic State still has a long sweep of territory along Syria's Euphrates valley and wide stretches of desert, despite recent losses to the SDF, the Syrian army and rebel groups.
Former residents in touch with relatives in the beleaguered city said at least 50 people, mostly civilians, were killed in escalating US-led strikes on residential areas inside the city.
More than 10 people were killed and dozens injured on Saturday when jets believed to belong to the US-led coalition hit the city's main Nour Street in the center of the commercial area, they said.
The other strike that killed at least 14 people was in the western neighborhood of Jazra, where jets targeted an internet cafe, according to Muhab Nasser, an exile from Raqqa in touch with relatives and friends.
To the west of Raqqa, the Syrian army and its allies have advanced into Islamic State territory and on Friday reached SDF lines near the town of Tabqa, 40km (25 miles) from the city.
The Syrian government has described the SDF's war against Islamic State as "legitimate" and said its military priorities are further east, suggesting it does not plan to confront the group now.