It has received reports ISIS has been instructing residents of Hammam al-Alil, about 30 kilometers (18.6 miles) south of Mosul, to hand over boys 9 and older since October 17, Ravina Shamdasani, spokeswoman for the UN refugee agency, said at press briefing in Geneva, Switzerland.
She said the boys were apparently being conscripted to fight on behalf of the terror group as it battles to hold on to its Iraqi stronghold in the face of a massive assault by Iraqi-led coalition forces.
Shamdasani said the United Nations continued to receive reports of mass killings by ISIS, including 50 of its own militants Monday at Mosul's Ghazlani military base for alleged desertion.
Iraqi troops are engaged in furious battles with militants to liberate the easternmost neighborhoods of Mosul after they breached the city limits for the first time in two years.
These forces have started storming the neighborhoods of al-Karama, al-Zahra, al-Qudes and al-Tahrir, Maj. Gen. Maan al-Saadi, commander of Iraqi counterterrorism special forces, told CNN on Friday.
Government troops struck ISIS positions early Friday, sending the militant group's ambulances rushing to the scene to extricate the wounded and dead.
A CNN team on the outskirts of Mosul heard loud clashes, bombing from air support and artillery fire. It also witnessed Iraqi Humvees racing toward neighborhoods in eastern Mosul.
Military radio chatter suggested that rooftop snipers, along with militants firing from civilian areas, were complicating the fight.
The Iraqi army has reported progress on the northern front, with the capture of areas near Tal Kayf.
However, Iraqi counterterror troops got an early taste of how dogged -- and bloody -- ISIS' defense of the city will be after encountering what one soldier described as "crazy resistance."
Three of the unit's soldiers were killed and seven wounded inside the neighborhood of Kirkokli.
The unit was met by two suicide vehicle bombs, and ISIS fighters emerged at close quarters with heavy machine guns and rocket-propelled grenades. The unit lost two Humvees, with damage to three others.
Iraqi military helicopters circled above the area, using heavy machine gun fire against suspected ISIS positions.
ISIS-launched mortars landed on residential areas, killing at least seven civilians.
Civilians continue to leave neighborhoods on the outskirts of Mosul despite efforts by Iraqi authorities to keep them in their homes. Many said it had become impossible to stay because there was no water or food.
Officials have warned that entering the city would likely trigger the fiercest fighting
yet and that the battle is expected to be fought street to street, or even house to house.
More human shields?
One challenge the liberators are facing is differentiating ISIS fighters from civilians. The terror group is reported to have brought tens of thousands of civilians -- mostly women and children -- into the city to be used as human shields, a tactic to ward off airstrikes
and ground assaults.
On Thursday night, ISIS militants ordered civilians to leave the town of Hammam al-Alil and head north into Mosul. The order via the loudspeakers of mosques came after the Iraqi military announced its troops had reached hills overlooking the town.
'New life breathed into us'
As forces broke through Mosul's border, the Iraqi army opened up a safe route for civilians to evacuate from the front line in al Intisar, Brig. Gen. Tahsin Ibrahim, an Iraqi Ministry of Defense spokesman, told CNN.
A CNN team on the ground saw hundreds fleeing on foot from the direction of Mosul and the village of Gogjali on the outskirts, risking ISIS' booby traps and gunfire. Many waved white flags to show they were civilians.
They gathered on the side of a road where the Iraqi military sent trucks and buses to pick them up and take them to a camp in the town of Khazir. The men were packed on the back of utility trucks, while buses carried women and children.
A woman wearing an Islamic niqab -- to hide her face and protect her family members still in Mosul -- told CNN how ISIS would shoot at civilians as they fled.
"It's very hard to describe our feelings right now. It feels like we have new life breathed into us -- a new soul," she said, holding her young daughter.
The UN migration agency said Friday that 22,224 people have been displaced since the offensive began October 17, with about 97% still in Nineveh province, of which Mosul is the capital.
These figures don't include more than 3,000 people displaced during the campaign who have since returned to their communities, the agency said.