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.

Forces were deploying within the prison facility and taking over buildings near its northern wing, said Farhad Shami, a spokesman for the Kurdish-led Syrian Democratic Forces. That's where dozens of IS militants have been holed up since Thursday. Over a dozen Kurdish fighters and more than 100 militants have been killed in clashes since the assault began.

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

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

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 prison's northern wing after combing the area. The SDF said in a statement that those building takeovers came after about 300 IS militants surrendered.

Witnesses in the city of Hassakeh, where the prison is located, said buses arrived at the prison apparently to transport militants who had turned themselves in to another location. Coalition helicopters were hovering overhead, a resident said.

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

The standoff followed a bold assault on Gweiran Prison on Thursday. Militants rammed vehicles through its walls, enabling a number of imprisoned fighters to escape and take hostages. Clashes have continued since then, 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 prison's northern wing.

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

Save the Children said in a statement Monday that audio testimony it and other groups have received suggests there have already been “multiple child deaths and casualties.” One such testimony was from a boy pleading for help, the group said, calling for the immediate evacuation of children from the prison.

“Reports that children have been killed or injured are tragic and outrageous,” said Sonia Khush, Save the Children's Syria response director. “All those involved in the fighting at (Gweiran) prison have a responsibility to protect these children from harm, and we urge them to take all possible steps immediately to ensure that these children can leave in safety.”

Many of the boys caught in the fighting have been held in Gweiran for almost three years, including children from Iraq, Syria and other countries, Save the Children said.

Khush called for foreign children to be repatriated with their families. “The international community cannot have the blood of any of these children on their hands,” she said.

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

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 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 Syrian Democratic Forces run 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 are 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.