ISIS executed almost 300 men and boys as the US-led coalition forces closed in on Mosul, an Iraqi intelligence source has told CNN.
Having captured 550 families from local villages for use as human shields, ISIS reportedly shot dead 284 males, some of them children, according to the source.
Zeid Ra’ad Al Hussein, UN High Commissioner for Human Rights, said his office had evidence of several instances over the past week where ISIS forced civilians to leave their homes in outlying villages and head to Mosul. It also had received reports that civilians suspected of disloyalty had been shot dead.
“There is a grave danger that (ISIS) fighters will not only use such vulnerable people as human shields but may opt to kill them rather than see them liberated,” he said.
The latest shootings came as the Iraqi army launched an operation to take Qaraqosh, a Christian town near Mosul, the last major city stronghold of Islamic State in Iraq, the military says.
Qaraqosh, about 20 kilometres south-east of Mosul, was emptied of its population in 2014, when Islamic State swept through the region.
Iraqi special units earlier this week captured Bartella, a Christian village north of Qaraqosh.
The offensive that started on Monday to capture Mosul is backed by the US-led coalition.
It is expected to become the biggest battle fought in Iraq since the US-led invasion in 2003. Islamic State also controls parts of Syria.
The army is also trying to advance on Mosul from the south, while Kurdish Peshmerga fighters are holding fronts in the east and north.
A Reuters photographer on the southern front on Friday saw plumes of smoke rising from a sulphur factory that was under the control of Islamic State, filling the air with toxic gasses.
It was not clear if the militants set it on fire to cover their retreat, or if it was damaged during the fighting.
Authorities in Kirkuk extended for a second day a curfew declared after Islamic State militants stormed police stations and other buildings in curfew in the northern oil city under control of Kurdish Peshmerga forces.
Iraqi Prime Minister Haider al-Abadi on Friday night ordered an army brigade to head to Kirkuk to assist the Peshmerga clear the remaining buildings still held by the militants.
Kurdish Peshmerga fighters took control of Kirkuk in 2014, after the Iraqi army withdrew from the region, fleeing an Islamic State advance through northern and western Iraq.
A total of 35 people have been killed since Friday in clashes, including four Iranian technicians who were carrying maintenance work in a power station north of the city, according to a hospital source.
The toll does not include the jihadists who were killed or who blew themselves up during the fighting.
Kurdish leaders say they will never give up the ethnically mixed city, to which they, as well as Turkmen and Arabs, lay claim.
Meanwhile the US Defense Department released the name of the serviceman who became the first American casualty in the mosul offensive,
Chief Petty Officer Jason C. Finan, 34, died from injuries suffered by an “improvised explosive device,” or roadside bomb, officials said.
The Californian, who was serving in a US Navy explosive ordnance disposal unit, was married with a seven-year-old son.