An English group that monitors the Syrian Civil War has reported that a coalition airstrike Monday on an Islamic State prison in eastern Syria killed dozens of prisoners.

The Syrian Observatory for Human Rights, claims the dawn strike killed 42 prisoners and 15 militants near Mayadeen in Dair Alzour province. The US coalition has said that it will investigate the claims.

Deirezzor24 news site, which is run by opposition activists, reported that 60 people had been killed, including civilians, Syrian rebel fighters and two militant guards. The prison, near the village of Tayibiya wa Maaharufa, housed those caught fighting Islamic State, including members of the coalition-backed Free Syrian Army and the Al Qaeda-linked group Hayat Tahrir al-Sham, once known as Nusra Front.

Mayadeen is a Euphrates River valley city about 30 miles southeast of Dair Alzour, an Islamic State stronghold frequently targeted in recent days by coalition aircraft as militant leaders fled there from Raqqah and Mosul.

A spokesman for the coalition acknowledged it had conducted strikes “on known ISIS command and control facilities and other ISIS infrastructure” in Mayadeen on Sunday and Monday. He said the group was investigating potential civilian casualties.

“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,” the spokesman said in a statement Tuesday. “This mission was meticulously planned and executed to reduce the risk of collateral damage and potential harm to noncombatants.”

The statement noted the coalition makes efforts to avoid civilian casualties when targeting strikes.

“The coalition strikes only valid military targets, after considering the principles of military necessity, humanity, proportionality, and distinction. We hold ourselves accountable with an open and transparent process to assess allegations of civilian casualties, and we publish these findings on a regular basis for the world to see,” the statement said.