DAMASCUS, June 27 -- At least 57 inmates and guards were killed when a U.S.-led airstrike targeted a prison under the control of Islamic State group in eastern Syria, a monitor group reported on Tuesday.

The airstrike targeted the prison in the city of Mayadeen in the countryside of Syria's eastern province of Deir al-Zour on Monday, said the Syrian Observatory for Human Rights.

It added that the prison used to be a house for one of the leaders of the al-Qaida-linked Nusra Front, which was captured by IS and turned into a prison, half of it for IS militants imprisoned by the group itself, while the other half is for civilians captured by IS.

The death toll could likely rise as there are many critically wounded people, the UK-based watchdog group added.

Around 100 inmates are imprisoned in that prison, said the Observatory.

The U.S.-led coalition has been active recently in striking IS positions in Raqqa and Deir al-Zour, as part of its resolve to support the Kurdish-led Syrian Democratic Forces (SDF) group in their push to dislodge IS out of Raqqa and parts of Deir al-Zour.