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geared up Monday for an assault on a prison in northeast that so-calledÌý group fighters stormed last week, sparking fears for the fate of hundreds of underage detainees.

IS fighters on Thursday rammed two explosives-packed vehicles into the Kurdish-run Ghwayran prison to launch a brazen jailbreak operation that has plunged the city of Al-HasakaÌýinto chaos.

The attack is the group's biggest since their once sprawling self-styled "caliphate" was defeated in 2019. It has already killed more than 150 people, most of them jihadists.

Fighting drove up to 45,000 residents of Syria's largest Kurdish city to flee their homes, according to the United Nations, but the violence receded on Monday with the presence of hundreds of children inside the prison complicating an assault.

The Britain-based Syrian Observatory for Human Rights said elite Kurdish forces had moved armoured vehicles inside the main prison yard, ahead of a possible attempt to storm the facility.

IS fighters were holed up in one building on the northern side of the prison after "dozens of jihadists surrendered to Kurdish forces" in recent hours, according to the war monitoring group.

Farhad Shami, a spokesman for the Kurdish-led Syrian Democratic Forces (SDF), said several IS members had handed themselves over following an SDF raid inside the jail.

An AFP correspondent in the area saw buses and military vehicles transferring what appeared to be IS fighters out of the prison.

The semi-autonomous Kurdish authorities running the region imposed a state of emergency across Hasaka, after at least seven civilians were caught in the crossfire and killed.

According to rights groups and the UN, more than 700 childrenÌýare thought be held in Ghwayran, a former school converted into a detention facility that is badly overcrowded, housing at least 3,500 suspected IS members.

Children 'trapped'

Eva Hinds, spokeswoman for UN children's agency UNICEF, said the plight of the trapped children, around 10 percent of whom are believed to be 15 or younger, was a source of "grave concern".

"The SDF initially allocated a special section for children," Hinds said. "Many of them have adult relatives inside and have since joined them in other sections."

Sara Kayyali, Syria researcher at Human Rights Watch, said: "These children are effectively trapped in Ghwayran prison."

HRW heard voice messages from an injured childÌýat Ghwayran who reported "there are dead bodies everywhere," Kayyali said.

"It's not clear if they have any kind of medical assistance," she added, explaining that most of the childrenÌýwere aged between 12 and 18.

Another group, Save the Children, said it had also received audio testimony indicating that "there have already been multiple child deaths and casualties".

Up to 25 people, mostly members of the Syrian Democratic Forces, were killed in the largest clashes between IS and the SDF since 2019

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It said the childrenÌýin Ghwayran are from dozens of foreign countries, as well as Syria and Iraq.

The SDF, the Kurdish authorities' de facto army, charged in a statement that IS "continues to hold children hostage and is using them as human shields to protect themselves" from an assault.

US-led coalition

The US forces based in the region, who were the main support in the Kurdish offensives that put thousands of jihadists into custody three years ago, deployed heavily in Hasaka.

US-led coalition helicopters flew overhead as a full curfew was enforced across the city, an AFP correspondent reported.

According to the Observatory war monitor, at least 154 people have been killed since the attack began late on 20 January.

Among them were 102 jihadists, 45 members of the Kurdish security forces and seven civilians, Observatory head Rami Abdel Rahman said.

Analysts saw the attack on one of the biggest prisons in the region as a sign that IS needs manpower to continue rebuilding following the demise of its "caliphate".

While it is unclear how successful IS will have been at springing fighters from Ghwayran, the operation marks a new step in the jihadist organisation's resurgence.

But analysts also argued that while IS has trumpeted the attack on its propaganda channels and that it might provide a morale boost for sympathisers, it would do little to change the military balance on the ground.