The Syrian Democratic Forces (SDF) on Monday said 121 of their fighters and prison workers died in the ISIS assault on Ghwayran prison in Hassakeh and during days of subsequent clashes with the militants.
About 374 ISIS fighters, including the initial attackers, were also killed and another 300 members of the extremist group have surrendered, the SDF said.
Four civilians were also killed during the assault, which began on January 20 and hit one of the largest detention facilities in Syria, the SDF said.
The attack has turned the city of Hassakeh into a conflict zone, with the UN saying more than 45,000 people have fled the fighting in city.
It was the biggest ISIS operation since the fall of its self-declared caliphate in 2019 and came as militants carried out deadly attacks in both Syria and Iraq that stoked fears the group may be staging a comeback.
At a press conference on Monday, SDF commander Nowruz Ahmad said the prison attack was intended to free several terrorist detainees, but was also part of a broader plot that ISIS had been planning for a long time.
Seized documents and confessions from some of the attackers show that the extremist group had planned attacks on other parts of Hassakeh, the town of Shaddada and areas in Deir El Zour in eastern Syria. Also planned were simultaneous attacks on Al Hol camp, which houses thousands of family members of ISIS militants.
“[ISIS] wanted to launch a massive attack on the region and once again to spread their terror and impose darkness on the people of the region and revive the terrorist organisation,” Mr Ahmad said.
The SDF played a key role in the 2019 territorial defeat of ISIS in its last remaining stronghold of Baghouz.
The siege began when Al Sinaa prison, which holds ISIS fighters, came under attack by suicide bombers, the SDF said.
A simultaneous attack on security forces from within the prison's walls began, with ISIS reinforcements arriving soon after.
“A large cargo car loaded with weapons and ammunition approached the prison gate so that terrorist detainees could get weapons as they escaped from the prison,” the SDF said in a statement on Monday.
On Sunday, US National Security Adviser Jake Sullivan praised the SDF for its efforts.
“Thanks to the bravery and determination of the SDF, many of whom paid the ultimate sacrifice, ISIS failed in its efforts to conduct a large-scale prison break to reconstitute its ranks,” he said in a statement.
“ISIS remains a global threat that requires a global solution. The United States remains committed to working with our partners in Iraq and north-east Syria, and the Defeat-ISIS Coalition, to counter the ISIS threat to our homelands.”
