The Syrian Observatory for Human Rights says 57 people have been killed in an air strike believed to have been carried out by the US-led coalition in an eastern Syrian town controlled by Islamic State.
A spokesman for the US-led coalition said it would quickly look into the matter.
The Observatory said the air strike took place on Monday at dawn, hitting a building in the town of al-Mayadeen that was being used as an Islamic State prison.
The Britain-based observatory says at least 15 IS jailers and fighters were also killed in the strike in the Deir El-Zour province.
The activist-run Deir Ezzor 24 media outlet says at least 60 civilians were killed. It says the building belonged to an al-Qaeda-linked commander before it was seized by the IS group in 2014.
