HASAKA, Syria — Kurdish-led forces regained full control of a prison in northeastern Syria on Sunday after a battle which spread to surrounding neighborhoods in the most intense urban combat involving American soldiers in Iraq or Syria since the self-declared ISIS caliphate fell in 2019.
“We announce the end of the sweep campaign in al-Sinaa Prison in Ghweran neighborhood in Hasaka and the end of the last pockets in which ISIS mercenaries were holed up,” the Syrian Democratic Forces, a Kurdish militia, said in a statement.
The U.S. Special Operations Joint Task Force said the militia had cleared the prison of “active enemy fighters” and was conducting recovery operations to make sure the area was fully safe. It said detainees were transferred to a more secure site.
The Syrian Democratic Forces, American partners in the fight against ISIS, did not say whether the last remaining gunmen in the prison had surrendered since Saturday or whether they had been killed. S.D.F. officials said Saturday that the gunmen were believed to be holding teenage detainees hostage.
Fighting in the past week has spilled into the residential areas of Hasaka near the prison.
New York Times journalists saw several dozen bodies, some dressed in orange prison jumpsuits, being carted away over the weekend by Kurdish militiamen near the prison, an indication of the scale of fighting in recent days.
On Sunday clearing operations continued in the Ghweran neighborhood around the prison to find ISIS sleeper cells. The day before, Kurdish-led counterterrorism forces backed by U.S. Special Operations troops went house to house in the narrow alleys of the neighborhood in the majority-Arab city.
Kurdish forces threw flash grenades into homes where they believed ISIS fighters were hiding as residents gathered in the streets.
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The latest round of fighting began this month after an attack by ISIS on the prison, which housed more than 3,000 ISIS members and almost 700 minors.
On Saturday the Syrian Democratic Forces said that about 30 ISIS fighters surrendered overnight but that the remaining militants in the prison were believed to be holding teenage detainees as human shields.
“We think there are cubs of the caliphate with them,” Farhad Shami, an S.D.F. spokesman, said in reference to the children forced by ISIS to become fighters.
The Kurdish militia has released conflicting information about the siege. On Wednesday it declared it had regained control of the prison after the United States launched airstrikes and sent in armored fighting vehicles to help retake it. On Thursday, it was clear that fighting with gunmen barricaded in prison buildings was continuing.
By Saturday, there were increasing signs that the battle was much fiercer than had initially been reported.
On the edge of the Ghweran neighborhood, journalists for The New York Times saw what appeared to be at least 80 bodies being transported in a small truck from the direction of the prison and being dumped in a pile on the road. Kurdish fighters heaved them one by one into the shovel of a yellow front-end loader, which moved them into a 40-foot gravel truck to be taken away for burial.
While some bodies were in prison jumpsuits, others were dressed in civilian clothing, as is also common among those held at the site. Almost all the corpses were intact and unbloodied, many of their faces and bodies black with soot.
A distraught fighter shouted at a Times photographer not to take photos.
“We know this is not right, but there are so many of them,” he said.
Hasaka, in the breakaway Kurdish-led region of Rojava, is surrounded by hostile Syrian forces and Turkish-backed troops who occupy northwestern Syria.
The region has been struggling with existential security threats, a lack of infrastructure and near financial collapse. Foreign countries have refused to repatriate ISIS fighters and their families, leaving Rojava to become a haven for the remnants of the self-declared ISIS caliphate, including thousands of accused fighters and tens of thousands of their family members.
The local administration in Rojava has long warned that it does not have the resources or the ability to run secure prisons and detention camps.
The United States maintains about 700 troops in Rojava as part of the U.S.-led coalition against the Islamic State. But until the prison siege the American forces for the most part conducted relatively routine missions that avoided the Russian military presence in the same area.
The S.D.F. said on Saturday that 13 of its fighters had been killed retaking the prison and securing the area, although that figure is probably higher. It has not released figures for the number of inmates killed in the fighting.
An official with the U.S.-led coalition who spoke on condition of anonymity because he was not authorized to speak publicly said it would take time to determine how many ISIS fighters had been killed.
S.D.F. officials have said prison inmates who were younger than 18 have been transferred to a new location. The minors were brought to Syria as young children with their parents.
An official with the Y.P.G., the main Kurdish faction, said most of the ISIS fighters who were still barricaded in the prison surrendered on Friday night after the Kurdish-led forces stormed the building.
“They told us they were surrendering and then they came out one by one and put their guns on the ground,” said Siyamend Ali, the Y.P.G. media director. He said some laid down suicide belts.
Hasaka has been under lockdown since the prison break on Jan. 20. Shops are shuttered and makeshift shelters house families displaced by the fighting. In some areas there has been no electricity or running water for more than a week.
In the Ghweran neighborhood on Saturday, a group of men and boys stood in an alley down the street from U.S. and Kurdish armored vehicles.
“It is an unbelievably bad situation,” said a laborer who would be identified only by his first name, Mohammad, because he feared speaking about ISIS. “The neighborhood has not been cleared properly yet and ISIS is using the rooftops to jump from one house to another.”