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Up to 25 people were killed in ongoing clashes between the Islamic State group (IS) and Kurdish-led Syrian Democratic Forces () after the former attacked a prison housing IS fighters in northeast on Thursday night.

This was the extremist group’s largest operation in Syria since the defeat of its self-proclaimed caliphate in March 2019.

IS launched an attack on the Ghuwayran prison in al-Hasakah province late Thursday, with bombs being launched at the prison’s walls while prisoners rioted inside.

The prison holds up to 3,500 suspected IS fighters, to the Syrian Observatory for Human Rights (SOHR).

Up to 18 of the 25 killed were from the Kurdish internal security forces (Asayish), while six IS fighters and one civilian were also killed, according to the SOHR.

The casualty count was neither confirmed nor denied by SDF spokesperson Aram Hanna, but he confirmed that some internal security forces members were killed in the operations and that updates would be forthcoming.

“The prison in Hasakah was attacked by a [IS] sleeping cell yesterday night. The perimeter of the prison has been secured and security forces have surrounded the [IS] cell, which took civilians as human shields,” Hanna told ������.

“Commando units are still carrying out anti-terrorist operations in coordination with internal security forces, and violent clashes between them and the [IS] cell are ongoing,” he added.

Some of the IS fighters who initiated the attack against the prison managed to slip away from security forces and were shooting from civilians’ homes, according to the Qamishli-based Rojava Information Center.

The SDF holds tens of thousands of suspected IS fighters in prisons across the swathe of northeast Syria it controls.

The SDF has appealed to the international community for assistance in the continued detention of these fighters, but has said that the level of support thus far has been insufficient.

Prisons are overcrowded and their living conditions are poor. Riots in these detention facilities happen sporadically, but are usually put down by the SDF, in coordination with the US-led international coalition to fight IS, fairly quickly.

IS has put out calls for members to launch attacks on prisons in Syria and Iraq to free their fellow fighters in the past, but no operations have reached the scale of Thursday’s attack.

The SDF has to individuals suspected of links to IS who were not convicted of violent crimes in an attempt to ease the overcrowding of camps and prisons.

However, a sizeable number of individuals remain locked up in detention centres with no clear plan for their release.

Though the SDF and the International Coalition eliminated the last traces of IS’s territorial control in March 2019, the group has continued to carry out guerrilla style attacks since.

Analysts have that the group is “regrouping” in both Syria and Iraq, with a UN report in 2021 estimating that 10,000 fighters remain active in the two countries.