But that is nowhere near the total that officials worry could be in danger once the fighting moves to the most populated areas across the Tigris on the west side of Mosul, which is still believed to be home to at least one million people. Reports from inside the city indicate that the Islamic State has set up elaborate defenses on the banks of the Tigris, including artillery pieces.

The United Nations said that militants were reported to have shot six civilians on Oct. 20 for keeping hidden SIM cards in defiance of an order to surrender them. A week later, a 27-year-old man was killed for keeping a cellphone, she added.

Among the witnesses to the recent killings was the sole survivor from a group of 50 former members of Iraq’s security forces who was abducted by militants, taken to the Mosul airport and shot. Although wounded, “he pretended to be dead, escaped, and we spoke to him,” she said.

Meanwhile, a mass grave discovered on Monday by Iraqi troops near an agricultural college in the town of Hamam al-Alil was only one among numerous sites of large-scale killings, Ms. Shamdasani said. The grave contained at least 100 corpses, but Islamic State fighters were also reported to have dumped bodies down a well and at a cement factory yard in the same town, and at several other locations including the Mosul airport and in the Tigris.

On a lengthening list of atrocities reported from Mosul, militants had deployed “sons of the Caliphate,” believed to be teenagers or younger, around the old town armed with explosive belts. They had also brought abducted women, some of them members of Iraq’s Yazidi minority, into the city to distribute them as slaves for their fighters.

Interviews with residents inside Mosul in recent days indicate a pattern of brutality by the Islamic State much like what the United Nations has been reporting, including an increase in the number of boys on the streets carrying rifles and swords. One resident told The New York Times that the Islamic State in recent days had executed 18 former security force members and driven their bodies in the back of a pickup truck to a freshly dug mass grave.

In addition to the Islamic State’s forcibly moving civilians into Mosul for use as human shields to deter attack, the United Nations said it had credible reports of the group’s fighters’ using chemical weapons and chemical agents like chlorine gas against advancing Iraqi and Kurdish forces. Ms. Shamdasani said that Iraqi troops entering the city had found large quantities of sulfur and ammonia stockpiled in civilian areas of the city.