Kurdish-led forces said they raided part of a prison seized by ISIS fighters in northeastern Syria and forced at least 300 of the militants to surrender on Monday.
The Syrian Democratic Forces (SDF), spearheaded by the Kurdish People's Protection Units (YPG), said militants were still holed up in other buildings, and plans were underway to clear the rest of the detention complex in Hasakah city.
"The operations to break into the prison have begun," one SDF source said. Another said allies in a US-led coalition were involved in the "ongoing operations" without elaborating. There was no immediate statement from ISIS.
At least 180 inmates and militants and 27 security forces have died since ISIS fighters attacked the jail on Thursday in a bid to free their members, officials have said.
The SDF initially said it had thwarted the breakout, but later acknowledged that inmates had taken over parts of the facility.
The United Nations children's agency (UNICEF) called late on Sunday for the evacuation of the nearly 850 children held in the complex with the militants and their families, saying their safety was at "immediate risk."
SDF officials declined to go into further details on their planned operation.
"Very sensitive developments are taking place regarding ending the mutiny of ISIS mercenaries," SDF spokesman Farhad Shami said in a tweet.
Arab tribal figures in touch with relatives said they feared the death toll was much higher than figures released by the SDF.
Residents said thousands of families had fled since security services raided the surrounding Ghweiran neighborhood to search for freed prisoners.
The jail is the largest among several publicly known ones where the SDF holds suspected militants and other detainees in what aids groups say are overcrowded and inhumane conditions.
The US-based 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.