Scores of civilians were reported on Tuesday to have been killed in what the US coalition was an attack on Islamic State "command and control" facilities in Syria.

The Syrian Observatory for Human Rights said at least 57 people were killed in air strikes on what it described as an "IS prison" in Mayadeen, a town near Deir Ezzor which the US says is being used by IS leaders fleeing their stronghold of Raqqa.

The coalition conducted strikes on known IS command and control facilities and other ISIS infrastructure - US coalition spokesman

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

The Syrian state-run TV station al-Ikhbariya meanwhile said the building had been used as a prison for a "large number of civilians".

A US coalition spokesman said he believed the targets were "command and control" facilities, but the coalition would investigate claims of civilian casualties.

"The coalition conducted strikes on known IS command and control facilities and other ISIS infrastructure," Joe Scrocca, the coalition's director of public affairs, told Reuters by email.

"The removal of these facilities disrupts [IS'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.

IS hideout

Mayadeen lies in the Euphrates Valley some 45km southeast of the provincial capital of Deir Ezzor. IS is believed to have moved most of its leadership there from Raqqa, according to US intelligence officials who spoke to Reuters.

Its propaganda and "foreign operations" centre are believed to be in the town, the officials said.

The coalition said the mission had been "meticulously planned" to reduce the risk of possible harm to non-combatants.

Ryan Dillon, a colonel and spokesman for the US-led coalition, said: "With every single allegation we will take it and look into it. If we are responsible for any civilian casualties we come forth and admit it."

He said Observatory reporting had previously been exaggerated.

The monitor last week said more than 420 civilians were killed by the US-led coalition in air strikes targeting IS militants inside two Syrian provinces over the past month.

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.

The monitor said the period between 23 May and 23 June saw the highest civilian death toll in coalition raids for a single month since strikes began on 23 September 2014.

Observatory head Rami Abdel Rahman said 222 civilians, including 84 children, were killed in the largely Islamic State group-held province of Deir Ezzor.