The threat came in a video released by the jihadists' Amaq propaganda agency, showing six men, their faces covered in bruises.

IS said the men were abducted members of the Iraqi police and the Hashed al-Shaabi paramilitary force, which played a key role in Iraq's war against the jihadists.

Iraq declared victory over IS in December after expelling the jihadists from all urban centres following a vast military campaign.

It has detained hundreds of women and children identified as jihadists or related to suspected IS fighters and has put many on trial.

More than 300 people -- including some 100 foreign women -- have been sentenced to death in Iraq for joining IS, while similar numbers have been handed life terms, judicial sources say.

Most of the convicted women are Turkish or from the former Soviet republics.