Clashes broke out on January 29 between Kurdish forces and Islamic State group fighters near a Syrian prison where dozens of jihadists are still holed up, a war monitor said.

An IS assault on the sprawling Ghwayran prison complex near the northeastern city of Hasakeh on January 20 sparked days of heavy fighting that has left some 270 people dead, and a bulldozer shovelled corpses onto a truck on January 29 to take some for burial.

The US-backed Syrian Democratic Forces (SDF) announced they had recaptured the prison on January 26, but that mop-up operations continued.

On January 29, there were “clashes in the vicinity of the prison between the Syrian Democratic Forces and Kurdish security forces on the one hand, and members of IS who are hiding in the area,” the Syrian Observatory for Human Rights said.

The war monitor, which relies on a network of sources inside Syria, said that four IS fighters took a local official and three civilians hostage for hours, holding them in a residential building near the prison.

Kurdish forces later freed the hostages and killed three IS fighters.

Earlier, US troops and Kurdish-led forces surrounded the building and deployed snipers on nearby rooftops, an AFP correspondent reported, adding that he heard intermittent shooting.

The SDF said on January 26 some 3,500 IS members had surrendered, but that holdout IS fighters had barricaded themselves inside the prison complex.

The IS gunmen are in “cellars that are difficult to target with air strikes or infiltrate,” the Observatory said.

SDF officials estimated that between 60 and 90 IS fighters were still in the basement and the ground floor above it.

Twenty of them surrendered on January 29, the Observatory said, adding that the SDF killed another five in an exchange of fire inside the prison.

The People’s Protection Units (YPG), the Kurdish armed group which is the principal component of the SDF, published a video showing several IS members who surrendered on January 29. An SDF source estimated their number at around 10.

Kurdish forces have repeatedly called for IS gunmen to surrender.

“Our forces have not used force with them so far,” said Farhad Shami, who heads the SDF’s media office.

On January 29, an AFP correspondent saw a truck carrying away piles of bodies from the area near the prison believed to be those of IS fighters.

A bulldozer dumped more bodies inside the truck, which then headed to an unknown location.

Shami said that the bodies would be buried in “remote, dedicated areas” under SDF control.

Kurdish-led forces have banned journalists from freely accessing the Ghwayran neighbourhood or approaching the prison since the start of the attack.

The fighting has killed at least 270 people, including around 189 jihadists, 74 Kurdish-led fighters and seven civilians, the Observatory said, adding that the death toll is likely to increase.

The violence prompted 45,000 people to flee Hasakeh, the United Nations said. Many took refuge in relatives’ homes, while hundreds more have been sleeping in the city’s mosques and wedding halls.

The war in Syria, which broke out in 2011, has killed close to half a million people and spurred the largest conflict-induced displacement since World War II.