The Syrian Observatory for Human Rights said on Tuesday dozens of people were killed in an air strike believed to have been carried out by the US-led coalition on an Daesh prison in eastern Syria.
The US-led coalition said it would look into the report.
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 a prison. It said at least 42 people were killed, identifying them as prisoners of Daesh.
Syrian state-run TV station al-Ikhbariya cited its correspondent in Deir al-Zor as saying coalition warplanes had destroyed a building in al-Mayadeen used as a prison by Daesh. In a news bulletin flashed on screen, it said the building had been used as a prison for a "large number of civilians".
Colonel Ryan Dillon, spokesman for the US-led coalition, said: "With every single allegation we will take it and look into it.
"If we are responsible for any civilian casualties we come forth and admit it," he said. He said Observatory reporting had previously been exaggerated.
Daesh is believed to have moved most of its leaders to al-Mayadeen in Syria's Euphrates Valley, southeast of the group's besieged capital Raqqa, two US intelligence officials have said.
Among the operations moved to al-Mayadeen, about 80 km (50 miles) west of the Iraqi border, were its online propaganda operation and its limited command and control of attacks in Europe and elsewhere, they said.