At least 57 people have been killed in airstrikes on a prison run by the Daesh terrorist group in eastern Syria, the Syrian Observatory for Human Rights said Tuesday.

"The strikes hit a Daesh jail in Mayadeen at dawn on Monday, killing 44 prisoners and 15 terrorists," Observatory chief Rami Abdel Rahman told AFP.

The jail included civilian prisoners as well as jailed members of the group. The prison usually houses more than 100 prisoners, the DPA reported.

Mayadeen lies in the Euphrates Valley some 45 kilometres (28 miles) southeast of the provincial capital of Deir Ezzor.