BEIRUT (NYTIMES) - One week after Islamic State in Iraq and Syria (ISIS) group fighters attacked a prison in northeastern Syria, where they have held out despite a heavy assault by a Kurdish-led militia backed by the United States, the terrorist organisation published its version of what had gone down.

In its official magazine, it mocked how many times in its history that its foes had declared ISIS to be defeated. Its surprise attack on the prison, it crowed, had made its enemies "shout in frustration: 'They have returned again!'" That description was not entirely wrong.

The battle for the prison, in the city of Hasaka, killed hundreds of people, drew in US troops and offered a stark reminder that three years after the collapse of the group's so-called caliphate, its ability to sow chaotic violence persists, experts said.

On Saturday (Jan 29), about 60 Islamic State group fighters still controlled part of the prison.

In Iraq, the group recently killed 10 soldiers and an officer at an army post and beheaded a police officer on camera.

In Syria, it has assassinated scores of local leaders, and it extorts businesses to finance its operations.

In Afghanistan, the withdrawal of American forces in August has left it to battle the Taliban, with often disastrous consequences for the civilians caught in the middle.

ISIS, which once controlled territory the size of Britain that spanned the Syria-Iraq border, is not as powerful as it once was, but experts say it could be biding its time until conditions in the unstable countries where it thrives provide it with new chances to expand.

"There is no US endgame in either Syria or Iraq, and the prison is just one example of this failure to work toward a long-term solution," said Dr Craig Whiteside, an associate professor at the US Naval War College who studies the group.

"It really is just a matter of time for ISIS before another opportunity presents itself. All they have to do is to hang on until then."

ISIS, whose history goes back to the insurgency after the US invasion of Iraq in 2003, reached the summit of its powers around 2015, when it ruled multiple cities in Syria and Iraq, attracted droves of foreign fighters from as far away as China and Australia, and ran a sophisticated propaganda machine that inspired or directed foreign attacks from Berlin to San Bernardino, California.

A military coalition, led by the US partnered with local forces in Syria and Iraq, managed to roll it back, until a Kurdish-led militia, the Syrian Democratic Forces, pushed it from its last patch of territory in early 2019.

Since then, the organisation has morphed from a top-down, military-style bureaucracy to a more diffuse and decentralised insurgency, according to terrorism experts and regional security officials.

But the importance of the prison as a target suggested that last week's attack would have been green lit "by the highest levels", Dr Whiteside said.

The group's ability to mobilise dozens of fighters and break into a prison that American and SDF officials long suspected was a target was an achievement and a propaganda coup no matter how the siege turns out.

A senior American official, speaking on the condition of anonymity, said the probable goal of the operation was to free some of the group's senior or mid-level leaders and fighters with specific skills, such as bomb-making.

The official estimated that perhaps 200 prisoners had escaped.

SDF officials have not confirmed that number and said they were still assessing the effect.

Before it attacked the prison in Hasaka last week, the Islamic State group in Syria was primarily operating in the country's sparsely populated east, where its fighters sought refuge in the desert to plot attacks on Syrian government and Kurdish-led forces, according to analysts and local residents.

From 2018 to 2021, it stepped up a campaign of assassinations of local leaders and tribal figures, killing more than 200, according to a study by DeirEzzor24, an activist network.

More recently, it has extorted local businesses for cash, spread flyers against the US-backed SDF and carried out a string of attacks on isolated checkpoints that has caused some to be abandoned, said senior Syria analyst Dareen Khalifa with the International Crisis Group.

"The reality is that it got worse in 2021, not because there were so many attacks on checkpoints, but there were enough attacks to make the internal security forces scared to man checkpoints," she said.

Other factors have contributed to the group's persistence, she said, citing the SDF's struggle to forge trusted relations with local residents in overwhelmingly Arab areas, porous borders, crushing poverty that makes it easier for the terrorists to smuggle weapons and people, and the area's overall instability.

What the Islamic State group has not been able to do since 2019 is control significant territory. The splashy operation in Hasaka, analysts said, does not change that.

"Contrary to popular opinion, that doesn't move the needle much, and it doesn't get them closer to reestablishing control over populations," Dr Whiteside said. That control, he said, is "their reason for being, why they call themselves 'the State'."

The prison attack was still one of the group's most ambitious since 2018, and it should not have come as a great surprise.

The prison was, in fact, a converted training institute beefed up with bars and other fortifications, not an ideal lockup for thousands of former fighters from a group that has historically relied on prison breaks to replenish its ranks.

And it was a known target.

Last month, the SDF media office released a video of a man identified as a captured ISIS commander saying he had been responsible for planning a foiled attack involving two car bombs and a bunch of armed commandos.

Their goal? To storm the prison in Hasaka that the group seized last week.