The civilians, which included a family of four, were forced into cages and were doused with oil before the extremists set them alight in the city of Hawija in the Kirkuk Province, reports say.
ISIS claimed the people were trying to flee to nearby Kirkuk city, which is controlled by government forces.
According to a source cited by al-Sumeiryeh news, the captives had their hands tied and were forced into metal cages before being set alight.
The number of ISIS militants fighting in Mosul has dwindled from thousands at the start of the government offensive more than eight months ago to a mere couple of hundred now, according to the Iraqi military.
Iraqi forces say they expect to reach the Tigris and gain full control of the city by the end of this week.
Mosul was the largest city captured by the Islamist extremists almost three years ago which was deemed to be the capital of the “caliphate” by the terrorists, which at its height covered large parts of Iraq and Syria.
Military spokesman Brigadier General Yahya Rasool, told state TV: “Victory is very near, only 300 meters separate the security forces from the Tigris.”
Mr Abadi declared the end of ISIS’ “state of falsehood” on Thursday, after the security forces took Mosul’s medieval Grand al-Nuri mosque.
It was from here that Islamic State leader Abu Bakr al-Baghdadi made his first and only video appearance, proclaiming himself “caliph” – the ruler of a theocratic Islamic state – on July 4, 2014.