UN human rights staff have received reports of atrocities being committed by Islamic State militants as Iraqi government forces close in on Mosul.
In one case, three women and three children were allegedly shot dead after trailing behind while being forced to march from one village to another.
Elsewhere, 15 civilians were reportedly killed and their bodies thrown into a river in an attempt to spread terror.
The UN says this reinforces fears that IS will use civilians as human shields.
About 30,000 Iraqi security force personnel, Kurdish fighters, Sunni Arab tribesmen and Shia militiamen, assisted by US-led coalition air strikes, launched the long-awaited offensive to retake Mosul eight days ago.
The operation is expected to take weeks, if not months, depending on how much resistance they face from the 3,000 to 5,000 militants believed to be inside Mosul.
Speaking after a meeting with coalition counterparts in Paris, the US Defense Secretary Ash Carter said the offensive was proceeding as planned.
He said the groundwork was being laid for the isolation of Islamic State in the Syrian city of Raqqa, the capital of the group's self-proclaimed caliphate.
"As we meet here, we are hoping to generate the local forces that will do so," he said, adding that the two campaigns might overlap.
Update on the battle for Mosul
- Fighting continues in villages to the north, east and south of the city
- The US-led coalition says it has carried out 32 air strikes on the area in a week
- Iraqi security forces say they have regained control of Rutba, in the western province of Anbar, which was attacked by IS militants on Sunday
A spokesman for the UN High Commissioner for Human Rights, Rupert Colville, told a news conference in Geneva on Tuesday that it was continuing to receive reports of depredations against children and women, as well as male civilians.
"It is however hard to immediately verify all the reports we are getting, so the following examples should be treated as preliminary and not definitive," he noted.
According to Mr Colville, human rights staff were informed that IS fighters had killed 15 civilians in the village of Safina, about 45km (28 miles) south of Mosul, and threw their bodies in the river.
Last Wednesday, militants reportedly tied six men from the same village to a vehicle by their hands and dragged them around, apparently because they were related to a tribal leader fighting IS alongside Iraqi government forces. They were also allegedly beaten with sticks and gun butts.
The following day, Iraqi security forces reportedly discovered the bodies of 70 civilians inside houses in the village of Tuloul Naser, about 35km south of Mosul.
On Saturday, militants shot dead three women and three children from Rufaila, also south of Mosul, because they were trailing behind a group of other people from the same village who were being forced by IS to relocate to another area.
The victims were lagging behind because one child had a disability. She was apparently among those killed.
And on Sunday, members of IS are reported to have killed 50 former Iraqi police officers they had been holding in a building outside Mosul.
"We very much fear that these will not be the last such reports we receive of such barbaric acts by [IS], and repeat our call on government forces and their allies to ensure their fighters do not take revenge on any of the civilians who escape from areas under IS control, and treat all suspected IS fighters they capture in accordance with international humanitarian law," said Mr Colville.
Iraqi security forces and allied Shia Muslim paramilitary groups have been accused of committing serious violations during previous operations against IS.
Last week, Amnesty International alleged they had subjected Sunni Arabs who had fled IS to arbitrary detention, enforced disappearance and unlawful killing after capturing them or placing them in custody.
Also on Tuesday, a Russian military spokesman accused the US-led coalition of killing more than 60 civilians and injuring 200 others in air strikes in and around Mosul over the past three days.
One jet bombed a school for girls in a southern district of the city on Friday, Lt Gen Sergei Rudskoi said.