"Children are facing multiple threats to their lives. Those stranded in the fighting are hiding in their basements, fearful of the next onslaught. Those who try to flee, risk being shot or wounded. Hundreds of civilians have already been reported killed and used as human shields," Peter Hawkins, UNICEF's representative in Iraq, said in a statement.
"Boys and girls who have managed to escape show signs of moderate malnutrition and carry psychosocial scars of the conflict, as a consequence of what they have been through," Hawkins added.
The UNICEF reiterated its call on all parties of the conflict in Mosul to protect the children from the ongoing violence.
"Children must be kept out of harms way no matter the circumstances," the representative said.
The Iraqi forces have been fighting inside the old city, but the troops are making slow progress due to the stiff resistance of IS militants and a large number of roadside bombs and booby-trapped buildings, in addition to IS snipers taking positions in the buildings and narrow alleys of heavily-populated neighborhoods.
According to recent UN reports, some 100,000 civilians are still trapped in the IS-held areas in the old city center and the adjacent al-Shifaa neighborhood.
Mosul, 400 km north of Iraq's capital Baghdad, has been under IS control since June 2014, when government forces abandoned their weapons and fled, enabling IS militants to take control of parts of Iraqs northern and western regions.