"More than 1,000 detainees, including some as young as 15, are being held without charge in horrendous conditions at makeshift holding centres in Anbar governorate, west of Baghdad," Amnesty International said in a statement.
In one of the facilities, run by the province's counter-terrorism agency, some 680 detainees were "packed together like sardines in a tin" without enough space "to stretch or lie down to sleep" and they were rarely allowed to go outside for a breath of fresh air, the rights group added.
The Daesh, which is outlawed in Russia and a number of other countries, has taken over large territories of Syria and Iraq, including a vast part of Anbar province, since 2014, declaring a caliphate on the territories under its control and committing numerous human rights atrocities.