BASHIQA, Iraq — New reports emerged Friday of public killings and other atrocities committed against Mosul residents by Islamic State militants, including dozens of civilians whose bullet-riddled bodies were hung from utility poles after they were accused of using cellphones to leak information to Iraqi security forces.

The United Nations human rights office said Islamic State fighters killed some 70 civilians in Mosul this week, part of a litany of abuses to come to light in recent days, including torture, sexual exploitation of women and girls, and use of child soldiers who were filmed executing civilians.

The revelations are the latest reports of Islamic State brutality as the group retreats into the dense urban quarters of Iraq’s second-largest city, forcing the population to go with them as human shields.

In its report, the U.N. human rights office in Geneva said the Islamic State shot and killed 40 people 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, speaking on condition of anonymity out of fear for his safety.

The violence is part of a disturbing pattern. As the army advances, Islamic State 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, 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.

Islamic State militants have boasted of the atrocities in grisly online photos and video. The United Nations has urged authorities to collect evidence of abuses to use in prosecuting the militants in tribunals.