About 850 children are caught in the crossfire as Kurdish-led Syrian Democratic Forces (SDF) aided by US troops attempt to storm the prison in Hasaka city after it was seized by militants last Thursday, leaving dozens dead.
"Every day counts. It's very hard to even imagine what atrocities these children are witnessing," Juliette Touma, UNICEF's Middle East and North Africa regional advocacy and communications head, told Reuters.
"The children's lives are in immediate risk," Touma said.
Dozens of Islamic State fighters escaped into the surrounding area in last Thursday's attack, which included detonating a car bomb near the prison gates, while other inmates took over part of the facility.
The SDF says the death tally now stands at around 200 inmates and 27 of its fighters, while over 550 militants have surrendered. Clashes continue with militants still holed up in some buildings.
The UN's children's agency said fighting must end immediately to allow safe passage for the 850 minors, some as young as 12. UNICEF could not verify if any of the children were among the casualties cited by the SDF.
The children were detained during US backed campaigns that finally drove Islamic State from its last territorial enclave in Syria in 2019.
US Human Rights Watch and other rights groups have long criticised the Kurdish-led forces who control large swathes of northeast Syria for holding children in overcrowded, makeshift prisons in inhumane conditions.
The Hasaka prison is the largest of several where the SDF holds thousands without charges or trial and includes civilians who resist forced conscription.
The mass detentions in recent years have fuelled growing resentment by Arab tribal members who accuse the Kurdish forces of racial discrimination, a charge denied by the Kurdish-led forces that rule their areas.
The fighting has also forced over 45,000 civilians, mostly women and children, to flee from their homes in districts near the prison.
"These families fled in such a hurry with almost nothing on them in harsh winter weather. Many have already been displaced and fled violence from other parts of Syria," Touma added.