Kurdish-led fighters clashed with ISIS in north-east Syria on Saturday near a prison that holds thousands of suspected extremists.
The Syrian Democratic Forces advanced slowly under the cover of US-led coalition air power in Ghwayran, Hassakeh governorate.
Fighting broke out on Thursday night when the extremist group unleashed its biggest attack in Syria since being defeated nearly three years ago.
More than 100 militants attacked Ghwayran Prison, sparking battles with US-backed Kurdish fighters in which dozens were killed.
Since their defeat, ISIS sleeper cells have carried out deadly attacks against the SDF as well as Syrian government forces on the west bank of the Tigris River.
“Attacking the detention facility was a top ISIS priority for more than a year,” said US State Department spokesman Ned Price.
“It is a credit to the diligent, capable efforts of the SDF and Global Coalition to Defeat ISIS forces that they were able to thwart numerous attacks during that period and limit the severity of this one.
“This attack highlights the importance of, and the need to fully fund, the Global Coalition to Defeat ISIS’s initiatives to improve the secure and humane detention of ISIS fighters, including by strengthening detention facility security. It also underscores the urgent need for countries of origin to repatriate, rehabilitate, reintegrate, and prosecute, where appropriate, their nationals detained in north-east Syria,” he added.
The group’s territorial control in Syria and Iraq, where they once declared their “caliphate”, was crushed by a years-long US-backed campaign, but it has killed scores of people in both countries in the past few months.
On Friday, the SDF’s top military commander, Mazloum Abadi, said ISIS had mobilised “most of its sleeper cells” to organise the prison break.
Farhad Shami, an SDF spokesman, said the militants arrived at Ghwayran Prison on Thursday evening armed with heavy machineguns and vehicles rigged with explosives.
The SDF brought more reinforcements into Hassakeh to regain control of areas taken by ISIS, residents said.
More civilians fled the areas of fighting as sounds of explosions echoed in the city and black smoke billowed from its southern edge.
Hassakeh governor Ghassan Khalil told Syrian state media that about 4,000 civilians had fled to areas controlled by Syrian government forces in the city and its suburbs.
Ghwayran Prison is the largest of about a dozen SDF-run centres holding suspected ISIS fighters, with about 5,000 inmates including commanders and figures considered among the most dangerous.
Another SDF spokesman, Siamand Ali, said most of the prison was under SDF control apart from a small part held by rioting prisoners.
“The battles are taking place on the edge of the prison,” and in nearby Zuhour, he said.
SDF fighters and US-led coalition aircraft attacked a technical academy building in which dozens of “Daesh terrorists took positions”, Mr Ali said.
The fighters were advancing slowly against ISIS gunmen in alleys and homes to protect the lives of civilians, he said.
About 45 ISIS militants were killed in the fighting, he said. A statement about SDF fatalities would be issued later, he said.
The Syrian Observatory for Human Rights, an opposition war monitor based in Britain, said that since Thursday night, 78 people had been killed — five civilians, 28 Kurdish fighters and 45 ISIS gunmen.
It said the SDF was using loudspeakers to call on the terrorists to surrender, but they refused to do so.