Kurdish forces geared up Monday for an assault on a prison in northeast Syria that Islamic State group fighters stormed last week, sparking fears for the fate of hundreds of under-age 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 Hasakeh into 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.

Map locating Hasakeh in Syria. Map locating Hasakeh in Syria. Photo: AFP / STAFF

Fighting drove some 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 US-backed Kurdish forces "started to infiltrate the part of the prison that remains under the control of IS fighters," after freeing several Kurdish fighters and prison staff held captive by the group.

IS fighters were holed up in one building on the northern side of the prison, according to the war monitoring group.

The Kurdish-led Syrian Democratic Forces said that around 300 IS fighters had surrendered to its forces following a raid on one of the buildings housing holdout jihadists.

Syrian Democratic Forces (SDF) cross a street in the northern Syrian city of Hasakeh on January 23 Syrian Democratic Forces (SDF) cross a street in the northern Syrian city of Hasakeh on January 23 Photo: AFP

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 Hasakeh, after at least seven civilians were killed in the crossfire.

According to rights groups and the UN, more than 700 minors are 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.

Syrian Democratic Forces (SDF) patrol a street in the northern Syrian city of Hasakeh on January 23 Syrian Democratic Forces (SDF) patrol a street in the northern Syrian city of Hasakeh on January 23 Photo: AFP

Eva Hinds, spokeswoman for UN childrens' agency UNICEF, said the plight of the trapped minors, 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."

A member of the Kurdish Syrian Democratic Forces (SDF) mans a checkpoint in the northern Syrian city of Hasakeh on January 23 A member of the Kurdish Syrian Democratic Forces (SDF) mans a checkpoint in the northern Syrian city of Hasakeh on January 23 Photo: AFP / Delil SOULEIMAN

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

HRW heard voice messages from an injured minor at 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 minors were aged between 12 and 18.

Syrians flee their homes in the Ghwayran neighbourhood of the northern city of Hasakeh on the third day of fighting between Islamic State group fighters and Kurdish forces after IS attacked a prison housing jihadists Syrians flee their homes in the Ghwayran neighbourhood of the northern city of Hasakeh on the third day of fighting between Islamic State group fighters and Kurdish forces after IS attacked a prison housing jihadists Photo: AFP

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

It said the minors in 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 a counter-offensive.

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 Hasakeh.

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

Pentagon Spokesman John Kirby said Monday that US forces had been involved in the fighting.

"We have conducted a series of strikes through this days-long operation to include the precision targeting of ISIS fighters who are attacking the SDF from buildings in the area," Kirby said.

US forces have also provided "limited" support on the ground to help establish secure areas, including positioning armored Bradley Fighting Vehicles on routes of access to the prison," he said.

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

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 IS sympathisers, it would do little to change the military balance on the ground.

In a statement issued Monday, the US-led 'Global Coalition' against IS said the jihadist attack was "a desperate attempt to replenish its depleted ranks and to regain lost momentum."

"The Global Coalition is determined to ensure it fails," it said.