Syrian Observatory for Human Rights says dozens killed in U.S.-led airstrike on ISIS prison

The Syrian Observatory for Human Rights said Tuesday that dozens of people were killed in an airstrike believed to have been carried out by the U.S.-led coalition on an ISIS prison in the eastern Syrian town of al-Mayadeen.

The coalition said it had carried out strikes on known ISIS targets on Sunday and Monday 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.

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The Islamic State in Iraq and Syria is believed to have moved most of its leadership to al-Mayadeen in Syria's Euphrates Valley, southeast of the group's besieged capital Raqqa, according to U.S. intelligence officials.

Among operations moved to al-Mayadeen, about 80 kilometres 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 airstrike took place 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 al-Zor 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 U.S.-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.

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"The coalition conducted strikes on known ISIS command and control facilities and other ISIS infrastructure in (Mayadeen), Syria, June 25 and 26," Col. Joe Scrocca, coalition director of public affairs, said in an email to Reuters.

"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," said Scrocca. "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."