BEIRUT (AP) — U.S.-backed Syrian Kurdish fighters began closing in Monday on the last wing of a prison in northeastern Syria that has been controlled by militants for days, the force and a war monitor said. The raid aims to end one of the most brazen attacks by the Islamic State group in years.

Farhad Shami, a spokesman for the Kurdish-led Syrian Democratic Forces, said forces are deploying with the prison facility and taking over buildings near the northern wing of the prison, where dozens of IS militants have been holed up since Thursday. Dozens have been killed in clashes since.

Shami said the force advanced after arresting dozens of militants who were in buildings nearby.

Journalists at the scene said Kurdish officials have asked them to step away from the prison vicinity earlier Monday, apparently in anticipation of a military operation.

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The Britain-based Syrian Observatory for Human Rights reported a buildup of Kurdish forces, backed by U.S. armored vehicles around the prison earlier Monday. Rami Abdurrahman, the head of the Observatory, said SDF and accompanying forces took over buildings near the northern wing after combing the area and after about 30 IS militants surrendered.

Eyewitnesses in Hasskeh said buses arrived to the prison, apparently to transport those who handed themselves in. Coalition helicopters were hovering overhead, a Hassakeh resident said.

Late Sunday, the Kurdish-led administration announced a weeklong curfew in Hassakeh starting Monday. Hundreds of city residents have fled the fighting.

Kurdish-led Syrian Democratic Forces fighters

Kurdish-led Syrian Democratic Forces fighters, take their positions at an alley near Gweiran Prison, in Hassakeh, northeast Syria, Sunday, Jan. 23, 2022. (AP Photo/Hogir Al Abdo)

The standoff followed the bold assault on the Gweiran prison in Hassakeh city Thursday. Militants rammed vehicles through its walls, enabling a number of militants to escape and hold hostages. Clashes have continued for days, including with militants holed up in adjacent residential areas. The U.S-led coalition carried out a number of strikes on suspected militants, who had taken control of the northern wing of the prison.

On Sunday, Shami said the militants were using hundreds of minors detained in Gweiran as human shields.

Abdurrahman said scores of minors, some as young as 15, had been moved to another prison facility to the south soon after the assault began.

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More than 3,000 suspected IS militants, including over 600 minors, are held in Gweiran, the largest of a dozen detention facilities in Syria housing militants.

The SDF said about 27 of its fighters were killed in the assault. Abdurrahman, of the Observatory, put the figure at 52, adding that about 100 militants were killed. The SDF said about 100 escaped and were arrested by the total number of fugitive is still not clear.

The SDF is running nearly a dozen facilities in northeastern Syria housing thousands of suspected IS militants, including foreigners, following the defeat of the extremists in 2019. The Kurdish-led administration has said the facilities were a strain on its resources and had appealed for countries to repatriate their nationals.

Thousands of IS family members and supporters are also held in displacement camps, in what amounts to detention facilities mostly for women and children.