US-led coalition air strikes on a jail run by the Islamic State group in eastern Syria killed at least 57 people, monitors said on Tuesday.

The UK-based Syrian Observatory for Human Rights said the air strike took place on Monday at dawn, hitting a building in the town of Mayadin, south of Raqqa, that was being used as a prison.

"The strikes hit an IS jail in Mayadin at dawn on Monday, killing 42 prisoners and 15 jihadists," Observatory chief Rami Abdel Rahman told AFP.

Many of the dead are thought to be civilians, as well as captured rebel fighters from the Free Syrian Army.

If the toll is confirmed, it would make it one of the deadliest single incidents since the US intervened in the Syrian war in 2014.

Islamic State is believed to have moved most of its leaders to Mayadin in Syria's Euphrates Valley, southeast of the group's besieged capital Raqqa, two U.S. intelligence officials have said.

Among the operations moved to Mayadin, about 50 miles west of the Iraqi border, were its online propaganda operation and its limited command and control of attacks in Europe and elsewhere, they said.

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