The air strike hit the prison in the city of Mayadeen in Syria's eastern province of Deir al-Zour on Monday, said the Syrian Observatory for Human Rights, Xinhua news agency reported. "This mission was meticulously planned and executed to reduce the risk of collateral damage and potential harm to non-combatants".
The Syrian Observatory, a UK-based group that monitors the country's six-year civil war through a network of sources on the ground, said the prison was hit at dawn on Monday by what were believed to have been coalition aircraft.
"This allegation will be provided to our civilian casualty team for assessment", he said.
Along with the Syrian Democratic Forces, led by Kurdish militias, the coalition is now taking part in an offensive against IS positions in al-Raqqa province, the terror organization's main stronghold in Syria.
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The coalition said as of the start of June of the 21,035 air strikes in Syria and Iraq since 2014, at least 484 civilians have been killed, although the figure is believed to be higher.
The Deirezzor24 news website said dozens of civilians and Syrian rebel fighters were detained there.
Among operations moved to al-Mayadeen, about 80 km (50 miles) west of the Iraqi border, are its online propaganda operation and its limited command and control of attacks in Europe and elsewhere, they say.
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.