Terror group Islamic State has shot dead more than 60 people this week and hung some of their bodies from poles after alleging they had collaborated with Iraqi troops.
The UN human rights office confirmed that ISIS killed some 60 civilians including torture, sexual exploitation of women and girls.
The revelations are the latest reports of ISIS brutality as the group retreats into dense urban quarters of Iraqi's second-largest city, forcing the population to go with them as human shields.
In its report, the UN human rights office in Geneva said that ISIS shot and killed 40 people on Tuesday after accusing them of "treason and collaboration," saying they communicated with Iraqi security forces by cellphone.
The bodies, dressed in orange jumpsuits, were hung from electrical poles in Mosul.
A day later, the extremists reportedly shot to death 20 civilians at a military base. Their bodies were hung at traffic intersections in Mosul, with signs saying they "used cellphones to leak information."
A Mosul resident, reached by telephone, said crowds have been watching the killings in horror. One victim was a former police colonel, he said.
As the army advances, ISIS militants have been rounding up thousands of people and killing those with suspected links to the security forces.
Soldiers last week discovered a mass grave in the town of Hamam al-Alil, 20 kilometers (12 miles) south of Mosul, containing some 100 bodies.
At the same time, the militants have gone door to door in villages south of Mosul, ordering hundreds to march at gunpoint into the city. Combat in Mosul's dense urban areas is expected to be heavy, and the presence of civilians will slow the army's advance as it seeks to avoid casualties.
ISIS militants have boasted of the atrocities in grisly online photos and video.
The United Nations has urged authorities to collect evidence of ISIS abuses of civilians to use in eventually prosecuting the militants in tribunals.
Iraqi troops are advancing from four fronts on Mosul, the last major ISIS holdout in Iraq. As Iraqi special forces battle in eastern neighborhoods of the city, Kurdish peshmerga forces are holding a line north of the city, while Iraqi army and militarized police units approach from the south.
Government-sanctioned Shiite militias are guarding western approaches.
With AP Inputs
