The fortified jail of Sosa is located near the Iraqi Kurdish city of Sulaimaniya and includes mainly militants of the hardline Islamist group who were captured during the fight against Daesh which started in 2014.
Kurdish security officials launched manhunt operations after the break-out late yesterday and 15 of the 21 were recaptured, two security officials said. The whereabouts of the other six remains unknown.
Although Sosa jail is located in the semi-autonomous Kurdish region of northern Iraq, the federal government has full control over the prison.
It was not clear how the inmates managed to escape the highly secured prison.
Daesh, which once occupied a third of Iraq’s territory, has been largely defeated in the country but still poses a threat along the border with Syria.
The group has resorted to guerrilla tactics since it abandoned its goal of holding territory and creating a self-sufficient caliphate that straddles Iraq and Syria.