U.S.-led coalition said it would look into the report
* Observatory says air strike hits prison, kills dozens
* Coalition says conducted air strikes in area, will assess claim
* Islamic State believed to have moved leadership to al-Mayadeen (Adds coalition statement on strikes in al-Mayadeen)
BEIRUT, June 27 (Reuters) - The Syrian Observatory for Human Rights said on Tuesday dozens of people were killed in an air strike believed to have been carried out by the U.S.-led coalition on an Islamic State prison in the eastern Syrian town of al-Mayadeen.
The coalition said it had carried out strikes on known IS targets in the town 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.
Islamic State 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 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.
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 al-Zor correspondent as saying coalition warplanes had destroyed a building in al-Mayadeen used as a prison by Islamic State 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 Islamic State in its de facto capital of Raqqa in northern Syria.
"The Coalition conducted strikes on known ISIS (Islamic State) 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 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. 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. (Reporting by Tom Perry; Editing by Richard Balmforth)
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