Iraq’s prime minister, Haider al-Abadi, has ordered the immediate execution of all convicted jihadists on death row in swift retaliation for the Islamic State’s killing of eight captives.

Abadi ordered the immediate execution of “terrorists condemned to death whose sentences have passed the decisive stage”, his office said, referring to convicts whose appeals have been exhausted.

More than 300 people, including about 100 foreign women, have been condemned to death in Iraq and 185 to life imprisonment for membership of the Islamic State group, a judicial source said in April. Most of the convicted women are Turkish or from former Soviet republics, while a Russian man and a Belgian national are also on death row.

Abadi vowed on Thursday to avenge the deaths of the eight Isis captives, a day after their bodies were found along a highway north of Baghdad.

“Our security and military forces will take forceful revenge against these terrorist cells,” he told senior military officials and ministers. “We promise that we will kill or arrest those who committed this crime.”

The corpses, found at Tal al-Sharaf in Salahuddin province, were decomposing and had been strapped with explosive vests, the army said.

They included six abductees who had appeared in an Isis video with badly bruised faces. Isis claimed they were Iraqi police officers or members of the Hashed al-Shaabi paramilitary force.

In the video, posted on Saturday by the Amaq propaganda outlet of Isis, the jihadists threatened to execute their captives unless Baghdad released Sunni Muslim women held in its prisons within three days.

But Abadi said that autopsies indicated the captives were already dead when the video was posted.

Iraq declared victory over Isis in December after expelling the jihadists from all urban centres, including second city Mosul, in a vast military campaign, but the Iraqi military has kept up operations targeting mostly desert areas along the porous border with Syria.

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