The executions on Thursday came shortly after Abadi ordered the "immediate" implementation of the death sentences of hundreds of convicted jihadists in response to the killings by IS.

"By order of Prime Minister Haider al-Abadi, 12 terrorists sentenced to death (whose appeals were exhausted) were executed on Thursday," a statement released by Abadi's office said.

It did not specify how they were executed but death sentences in terrorism-related cases are usually carried out by hanging.

More than 300 people, including around 100 foreign women, have been condemned to death in Iraq and hundreds of others to life imprisonment for membership of IS, a judicial source said in April.

Abadi, who has faced charges of failing to respond in force to IS, on Thursday ordered "the immediate punishment of terrorists condemned to death" whose appeals have been exhausted, his office said.

He vowed to avenge the deaths of the eight IS 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," he added.

Iraq declared victory over IS in December after expelling the jihadists from all major towns and cities in a vast offensive.

But the Iraqi military has kept up operations targeting mostly remote desert areas where jihadists have continued to carry out attacks.