More than 100 IS fighters late Thursday stormed the Kurdish-run Ghwayran prison in the northern Syrian city of Hasakeh using suicide bombs and heavy weapons, freeing prisoners and setting off days of clashes.
Fighting continued for a fifth day Monday, with more than 700 minors thought to be inside the jail, who the UN childrens' agency warn they could be "harmed or forcibly recruited".
Here are assessments by analysts as to whether it marks a turning point.
How sophisticated was the attack?
IS expert Aymenn Jawad al-Tamimi called it the group's "most sophisticated operation" in Syria since its so-called caliphate fell in 2019 in the riverside hamlet of Baghouz.
In a statement from its Amaq propaganda arm, IS said it carried out a "wide and coordinated" operation that started with the detonation of two truck bombs at the outer gates of a prison housing fellow jihadists on Thursday night.
IS fighters then simultaneously attacked several points around the prison, the statement said, while inmates inside raided a weapons storage room and took several captives.
The group claimed hundreds of inmates were set free after jihadists infiltrated parts of the facility, but it remains unclear how many managed to escape.
More than 150 people, most of them jihadists but also dozens of Kurdish fighters, have been killed according to the Britain-based Syrian Observatory for Human Rights monitor, which relies on a network of sources inside the country.
On Monday, the Kurdish-led Syrian Democratic Forces (SDF), backed by US-led coalition forces, locked down Hasakeh in a bid to trap IS fighters.
What's the goal?
Depending on the number of jihadists who make it past the Kurdish security cordon, IS "capabilities could be bolstered to carry out more sophisticated, large-scale operations", Tamimi said.
But he added that this would in no way compare to the group's capabilities in 2014, when it managed to overrun swathes of land across Syria and neighbouring Iraq.
"There is still a long way off before getting anywhere near to that and seizing large amounts of territory," Tamimi said.
The Ghwayran prison attack, however, is a step in that direction, according to Nicholas Heras of the Newlines Institute in Washington.
"IS needs to spring its army from jail," Heras said. "We can expect more of these types of operations in the future, especially because the SDF is under-resourced to defend prisons where IS fighters are incarcerated."
A UN report last year estimated that around 10,000 IS fighters remained active across Iraq and Syria, many of them in Kurdish-controlled areas.
Kurdish authorities say some 12,000 IS suspects of more than 50 nationalities are being held in several of their jails in Syria's northeast.
Prisoners' wives and children are languishing in overcrowded displacement camps in the region, that are increasingly becoming a hotbed for extremism and radicalisation.
Pieter Van Ostaeyen, a historian and Syria expert, said surviving IS fighters "will likely disappear again in the desert" after the attack.
"I do not see any immediate threat of them conquering swathes of territory again," he told AFP.
The US-led coalition battling IS said the Ghwayran assault would leave the group weaker because it incurred heavy casualties.
"While Daesh remains a threat, it is clearly no longer the force it once was," the coalition said Sunday, using an Arabic acronym for the extremists.
Charlie Winter, an IS expert and conflict researcher, said that while the attack in Hasakeh province may not have an immediate impact on the ground, symbolically it was "absolutely huge".
"I haven't seen IS's online supporters excited like this in years," said Winter, who monitors the jihadist group's social media channels.
"This kind of ability to animate supporters has revitalised a lot of the community," especially in Syria, he said.
Security measures will prevent most of the jihadists and inmates from making it out of the province, Winter told AFP.
"But the symbolism and the kind of momentum that this attack will give to the IS insurgency in general could stand to have its own material implication," Winter said. "This is a demonstration of its capability, of its defiance."