Monitoring group Syrian Observatory for Human Rights earlier said that at least 57 people were killed at a jail run by Daesh in eastern Syria by air strikes carried out by the US-led coalition. "This mission was meticulously planned and executed to reduce the risk of collateral damage and potential harm to non-combatants".
The jail included civilian prisoners as well as jailed members of the group.
Ryan Dillon, a colonel and spokesman for the US-led coalition, said: "With every single allegation we will take it and look into it".More news: Evacuations ordered for fire on California coast
At least 15 ISIL fighters were also killed in the raid, the UK-based monitor, which tracks developments in Syria's long-running conflict via a network of contacts on the ground, said.
ISIL is believed to have moved most of its leadership to Mayadeen, southeast of Raqqa, according to USA intelligence officials.More news: McCarthy found guilty of 2nd-degree murder of Bella Bond
Among the operations moved to al-Mayadeen, about 80km west of the Iraqi border, were its online propaganda operation and its limited command and control of attacks in Europe and elsewhere, they said.
"This disrupts [IS's] ability to conduct terrorist attacks in both in Iraq, Syria, against coalition partner forces, and also against our homelands", he told the BBC.
Last month's civilian death count of 427 from US-led coalition air strikes is more than double the previous 30-day toll from April.More news: Avis teams up with Waymo on self-driving car program