HASAKA, Syria (NYTIMES) - Kurdish-led forces regained full control of a prison in north-eastern Syria on Sunday (Jan 30) after a battle that spread to surrounding neighbourhoods in the most intense urban combat involving American soldiers in Iraq or Syria since the self-declared Islamic State 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, using an alternative name for the Islamic State group.

The US 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, US partners in the fight against the Islamic State, did not say whether the last remaining gunmen in the prison had surrendered since Saturday or whether they had been killed. SDF 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 neighbourhood around the prison to find Islamic State sleeper cells. The day before, Kurdish-led counterterrorism forces backed by US Special Operations troops went house to house in the narrow alleys of the neighbourhood in the majority-Arab city.

Kurdish forces threw flash grenades into homes where they believed Islamic State fighters were hiding as residents gathered in the streets.

The latest round of fighting began this month after an attack by the Islamic State group on the prison, which housed more than 3,000 Islamic State members and almost 700 minors.

On Saturday, the SDF said that about 30 Islamic State 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 SDF spokesperson, said in reference to the children forced by the Islamic State 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 US 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 neighbourhood, 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.

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 Islamic State fighters and their families, leaving Rojava to become a haven for the remnants of the self-declared Islamic State 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 US maintains about 700 troops in Rojava as part of the US-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 SDF 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 numbers of inmates killed in the fighting.

An official with the US-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 Islamic State fighters had been killed.

SDF 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 YPG, the main Kurdish faction, said most of the Islamic State fighters who were still barricaded in the prison surrendered 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 YPG media director. He said some laid down suicide belts.