Monitor says suspected US-led raid killed dozens of people, mostly civilian inmates, at an ISIL-run jail in Deir Az Zor.

A suspected US-led coalition air raid on an ISIL-run prison in eastern Syria has killed at least 57 people, according to a monitoring group.

The Syrian Observatory for Human Rights said on Tuesday that most of the dead in the town of al-Mayadeen in Deir Az Zor province were civilian inmates.

At least 15 ISIL fighters were also killed in the raid, said the UK-based monitor, which tracks developments in Syria’s long-running conflict via a network of contacts on the ground.

Al-Ikhbariya, a Syrian state-run TV station, also cited its Deir Az Zor correspondent as saying coalition fighter jets had destroyed a building in al-Mayadeen used as a prison by ISIL to hold a “large number of civilians”.

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An activist-run media outlet in Deir Az Zor also reported the hit, which it said took place at dawn on Monday.

The US Central Command confirmed to the Associated Press news agency on Tuesday that it struck ISIL facilities in al-Mayadeen on June 25 and 26, but made no direct reference to the alleged ISIL prison.

CENTCOM said it would assess the allegations that civilian prisoners were killed, and it would publish the results of its assessment in its monthly civilian casualty report.

It said the Mayadeen mission was “meticulously planned and executed to reduce the risk … to non-combatants”.

The US-led coalition is supporting an alliance of Kurdish and Arab fighters in their assault on ISIL in its de facto capital of Raqqa in northern Syria.

“The removal of these facilities disrupts ISIS’s ability to facilitate and provoke terrorist attacks against the coalition, our partner forces and in our homelands,” Colonel Joe Scrocca, coalition director of public affairs, said in an email to Reuters news agency.

ISIL is believed to have moved most of its leadership to Mayadeen, southeast of Raqqa, according to US intelligence officials.

Among operations reportedly moved to Mayadeen, about 80km west of the Iraqi border, are its online propaganda operation and its limited command and control of attacks in Europe and elsewhere, they said.