Members of the Syrian Democratic Forces are pictured in the northern Syrian city of Hasakeh on January 24, 2022. AFP
Around 100 Islamic State fighters attacked Ghwayran prison in the northeastern city of Hasakeh on January 20, in their biggest military operation since their "caliphate" was defeated in 2019.
The ensuing clashes with the Kurdish forces running northeastern Syria have left more than 160 people dead, including 45 in Kurdish ranks, according to the Syrian Observatory for Human Rights.
Some of the estimated 3,500 IS prisoners inside the facility have already been bused out to other detention centres in recent hours but it was unclear how many remained holed up inside Ghwayran.
Some of the hundreds of minors detained in the prison were transferred on Monday, the Observatory said.
"If there is no deal for a swap, there will be a massacre, hundreds of people will be killed," Observatory head Rami Abdel Rahman told AFP.
The Syrian Democratic Forces, the Kurdish administration's de-facto army, have not confirmed reports that several prison guards were being held by IS fighters.
SDF forces operating with air support from the US-led coalition present in the region have deployed elite units and armoured vehicles in and around the converted school that became one of the world's largest IS prisons.
An assault has looked imminent since early Monday but the Observatory said Kurdish forces were reluctant to move in due to the presence of hostages inside.
The SDF is counting on the besieged jihadist fighters running out of ammunition and supplies, Abdel Rahman said.
He said talks were taking place for some of the Kurdish forces and prison staff trapped inside to be freed in exchange for medical treatment for wounded jihadist fighters.
The Observatory put the number of hostages held inside the prison at 27 with around 40 people whose whereabouts are unknown.