Many of the victims suffered grisly torture before their death, the organisation said.

The New York-based group said the Iraqi government had 'done nothing' to stop the killings, and that some of the people of the organisation's researchers interviewed had accused Iraqi security forces of assisting in the attacks.

The group said a man it interviewed in April described being tied from the ceiling and gang-raped by interior ministry officers over the course of days.

'They beat me all over my body,' the man, identified in the report by the pseudonym 'Nuri', told the group's researcher. 'When they had me hanging upside down, they used me like a punching bag ... They used electric prods all over my body.'

'Then they raped me,' he continued. 'Over three days. The first day, 15 of them raped me; the second day, six; the third day, four. There was a bag on my head every time.'