Airstrikes by a U.S.-led coalition fighting Islamic State killed at least 29 civilians including 14 children in 24 hours in the jihadists' Syrian capital of Raqqa, a war monitor said late on Tuesday.
The dead included a family of 14 who had fled to Raqqa from the desert city of Palmyra, the Britain-based Syrian Observatory for Human Rights said.
A spokesman for the U.S.-led coalition could not immediately be reached for comment.
The coalition has previously said it strenuously tries to avoid civilian casualties and investigates all reports that its strikes have killed civilians.
The coalition said in July that its strikes had killed at least 600 civilians in both Iraq and Syria since it began operations in 2014, a figure that is far lower than those given by independent monitors.
The Syrian Democratic Forces (SDF), an alliance of coalition-backed groups, began its assault on Raqqa in June after a months-long campaign to isolate the city.
The U.S.-led coalition has helped it with airstrikes and some artillery support from special forces, as well as by arming the Kurdish-led SDF.
Islamic State has retreated over the past 18 months from much of its Syrian territory under attack from three rival forces: the SDF, the Syrian army, and Turkey-backed rebels.