12 Islamic State group terrorists, who were on death row, were executed Thursday on the order of Iraqi Prime Minister Haider al-Abadi, his office said late Thursday. The executions were carried out in retaliation to the group's killing of eight hostages. 

The executions came shortly after al-Abadi ordered the "immediate" implementation of the death sentences of hundreds of convicted terrorists linked to ISIS. It was not mentioned how the 12 ISIS militants were executed. However, those on death row over terrorism-related charges are usually executed by hanging.

"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 his office said. Al-Abadi vowed 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."

According to reports in April, more than 300 male and female ISIS supporters, including foreigners, were sentenced to death by Iraqi courts for alleged membership with the group. Earlier this week, Iraq rejected demands by ISIS to release Sunni female prisoners.

In December, Iraq said that it had won the battle against ISIS but military operations continued in mostly remote desert areas where terrorists had been carrying attacks. Iraq last week declared complete victory over ISIS after more than three years of fighting. 

"All Iraqi lands are liberated from terrorist [ISIS] gangs and our forces completely control the international Iraqi-Syrian border," Lt. Gen. Abdul-Amir Rasheed Yar Allah had said in a statement.

The only video ISIS released since its defeat last year was that of eight hostages, who were forced to identify themselves as policemen and militia fighters. Brigadier-General Yahya Rasool, a representative for the Iraqi army’s Joint Operations Command (JOC), said on Monday that he had seen the ISIS video, released on June 23. Efforts were made to find the hostages, but Iraqi security forces later found the captives’ bodies, dismembered and dumped by the roadside on Wednesday in Salahuddin Province’s Tuz Khurmatu district.