Kurdish-led Syrian Democratic Forces (SDF) and US troops attempted to storm the Gweiran prison, in the city of Hassakeh, after it was seized by militants last Thursday.

Around 850 children are thought to have been caught up in the clashes, which have lasted six days, UNICEF said.

"Every day counts. It's very hard to even imagine what atrocities these children are witnessing," UNICEF's Middle East and North Africa regional advocacy and communications head, Juliette Touma said.

"The children's lives are in immediate risk."

More than 220 people have been killed and around 550 militants have surrendered so far, according to the SDF.

The fighting has also forced over 45,000 civilians, mostly women and children, to flee from their homes.

"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," Ms Touma added.

Dozens of inmates escape while others take over prison

Dozens of IS inmates managed to escape during the attack, which has seen a car bomb detonated and vehicles used to ram through the prison walls.

Other prisoners have taken over part of the facility and some are still holed up in nearby buildings.

The UN's children's agency said fighting must end immediately so the children, some of which are as young as 12, can leave safely.

They were all detained during the US-backed campaigns that drove IS from its last territorial enclave in Syria back in 2019.

US Human Rights Watch and other organisations have long criticised the Kurdish-led forces for holding children under inhuman conditions in makeshift prisons.

The Gweiran prison is the largest of several where the SDF holds thousands without charges or trial and includes civilians who resist forced conscription.

It holds around 3,500 IS prisoners and is one of many in the region housing former members of the group.