At least 40 people, mostly civilians, were killed on Saturday in US-led coalition strikes on an ISIL stronghold in eastern Syria, local residents and a foreign monitor said.
ISIL's Amaq news agency quoted a medical source as saying 40 people had been killed, a claim which was also made by Syrian state media.
The Britain-based Syrian Observatory for Human Rights said the strikes in Deir Ezzor province killed 36 ISIL family members including 17 children, while another seven victims had not yet been identified as either civilian or ISIL fighters.
The coalition's spokesman Colonel Sean Ryan confirmed there had been strikes but denied civilians were killed.
"The coalition takes great measures to identify and strike appropriate ISIL targets in order to avoid non-combatant casualties," he said.
The fighting is the latest phase of efforts by the US-led coalition and the Syrian Democratic Forces (SDF) to clear ISIL from its last foothold east of the Euphrates River following last year's defeat of the group in Raqqa, its Syrian headquarters.
Relatives of families from the area and Syrian state media say dozens of people have been killed in the last two weeks from US coalition airstrikes in the town of Hajin and areas around it.
Just over a week ago, the Syrian government protested to the UN about a coalition airstrike which it said killed 26 civilians in Hajin.
