BEIRUT (Reuters) - The Syrian Observatory for Human Rights said on Tuesday (June 27) that dozens of people were killed in an air strike believed to have been carried out by the US-led coalition on an Islamic State in Iraq and Syria (ISIS) prison in the eastern Syrian town of al-Mayadeen.
The coalition said it had carried out strikes on known ISIS targets in the town on Sunday (June 25) and Monday (June 26) - the day the Observatory said the prison was hit, killing 57 people.
The coalition said the mission had been "meticulously planned" to reduce the risk of possible harm to non-combatants. It added it would assess the Observatory's allegation.
ISIS is believed to have moved most of its leadership to al-Mayadeen in Syria's Euphrates Valley, south-east of the group's besieged capital Raqqa, according to US intelligence officials.
Among operations moved to al-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 say.
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.
Separately, Syrian state-run TV station al-Ikhbariya cited its Deir Ezzor correspondent as saying coalition warplanes had destroyed a building in al-Mayadeen used as a prison by ISIS to hold a "large number of civilians".
The US-led coalition is supporting an alliance of Kurdish and Arab fighters in their assault on ISIS in its de facto capital of Raqqa in northern Syria.
"The coalition conducted strikes on known ISIS command and control facilities and other ISIS infrastructure in (Mayadeen), Syria, June 25 and 26," Colonel Joe Scrocca, coalition director of public affairs, said in an e-mail to Reuters.
"The removal of these facilities disrupts ISIS' ability to facilitate and provoke terrorist attacks against the coalition, our partner forces and in our homelands. This mission was meticulously planned and executed to reduce the risk of collateral damage and potential harm to non-combatants."
"This allegation will be provided to our civilian casualty team for assessment," he said.