After having been consigned to the wilderness for the past three years, the rabidly fanatical Islamic State of Iraq and Syria (Isis) is on the offensive again with the attack on a prison in Syria. At least 120 people have been killed in the continuing battles between the US-backed, Kurdish-led forces and Isis fighters.

The UK-based Syrian Observatory for Human Rights has said that “at least 77 Isis activists and 39 Kurdish fighters, including internal security forces, prison guards and counter-terrorism forces have been killed” in the attack that started on Thursday. The Caliphate has claimed responsibility for the prison break. A video released on Saturday purported to show armed men infiltrating the Kurdish-run Ghwayran jail in Hasakeh city and raising the group’s black flag. At least seven civilians were also killed in the fighting.

The Kurdish-led Syrian Democratic Forces (SDF) said on Sunday that it had tightened its siege, aided by US-led troops, and claimed that 17 of its soldiers had been killed. The SFF had initially said it had thwarted the breakout and arrested 89 fighters sheltering nearby, but later acknowledged inmates had taken over parts of the facility. The militia has recaptured 104 prisoners who escaped, but the number of those who broke out had not been determined.

The attackers detonated a car bomb near the prison gates, helping dozens of inmates flee into the Ghwayran district. By all accounts, it was a calibrated prison-break. The SDF’s top military commander, Mazloum Abadi, said Isis had mobilised “most of its sleeper cells” to organise the operation.

In Washington, the Pentagon confirmed that the coalition against Isis had carried out air attacks in support of the SDF as it sought to put an end to the prison-break. Detention centres hold thousands of suspected Isis activists who were arrested after they were defeated with US support in northern and eastern Syria. It was not clear how many inmates were in the Ghwayran jail, which is the biggest facility where the SDF keeps detainees, said to number in the thousands.

Human Rights Watch estimates the SDF holds about 12,000 men and boys suspected of Isis affiliation, including 2,000 to 4,000 foreigners from about 50 countries. The battles have triggered a civilian exodus from neighbourhoods around Ghwayran with families fleeing in the harsh winter cold as Kurdish forces closed in.

“Thousands have left their homes near the prison, fleeing to nearby areas where their relatives live,” Sheikhmous Ahmed, an official in the autonomous Kurdish administration, said. It is evident that while Isis lost large swathes of territory in Syria in 2019, it retains at least some support, and has kept its sleeper cells on call. While militarily outnumbered, these cells have the potential to carry out guerrilla-style attacks. Last week’s attack on the prison was a display of this prowess and sends a message to the forces arrayed against it that Isis’ potential for mischief, while blunted in the region, still exists.