Kurdish-led forces said they had forced at least 300 militants to surrender after a deadly ISIS raid on the Ghwayran prison in Syria’s north-eastern city of Hassakeh.
Syrian Democratic Forces spokesman Farhad Shami said the operation was going “as planned”.
On Monday, the SDF again called for ISIS members holed up in the detention complex to surrender.
“We know that you have an internet connection and see your setbacks. We know that your leaders use you as firewood to get warm,” the SDF media centre wrote in a post aimed at the ISIS militants.
“We inform you that your leaders are negotiating with us to secure a way out, leaving you to death.”
The city was locked down on Monday to trap the ISIS fighters, as the death toll from the fighting grew to more than 150.
The SDF said ISIS was using hundreds of children as human shields.
The UN says more than 45,000 residents in Hassakeh have fled the fighting as it enters a fifth day.
Unicef, the UN children's agency, called for the protection of about 850 minors detained inside the jail, some as young as 12. The agency said they could be “harmed or forcibly recruited” by ISIS.
ISIS claims to have killed and injured several SDF members.
The prison is the largest used by the SDF to holds suspected militants and other detainees in what aid groups say are overcrowded and inhumane conditions.
Human Rights Watch says the SDF holds a total of about 12,000 men and boys suspected of ISIS affiliation, including 2,000 to 4,000 foreigners from almost 50 countries.
Elders say support for ISIS has grown with rising local resentment against the Kurdish-led administration they accuse of discriminating against the majority Arab population it rules, many of whom reject its policy of forcible conscription.
The Syrian Kurdish forces deny the accusations and say their rule redresses injustices as an oppressed minority during decades of Arab nationalist rule from Damascus.