Daesh fighters arrested after they attacked Gweiran Prison, in Hassakeh, northeast Syria, on Friday. AP

The Daesh group unleashed its biggest attack in Syria since the fall of its “caliphate” three years ago. More than 100 militants assaulted the main prison holding suspected extremists, sparking a battle with US-backed Kurdish fighters that continued 24 hours later and left dozens dead on Friday.

Daesh said via Telegram its militants had waged a wide-scale attack on the prison in northeastern Syria since Thursday night, and that clashes were ongoing nearby and in other neighbourhoods.

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SDF General Commander Mazloum Abdi said on Twitter, however, that "the area around the prison was completely surrounded and all fugitives were arrested."

Across the border in Iraq, gunmen stormed an army barracks north of Baghdad before dawn Friday while soldiers inside slept, killing 11 before escaping — the deadliest attack in months on Iraq’s military.

Kurdish-led Syrian fighters gatherer around a body of a Daesh gunman in Hassakeh, northeast Syria. AP

In Washington, the Pentagon confirmed the US-led coalition had carried out air strikes in support of the SDF as it dealt with the prison break.

A military source in the Kurdish-led forces said they killed 23 Daesh militants, including foreigners, in the clashes after the militant group attempted to free prisoners from the jail in Hasaka.

The attack in Syria targeted Gweiran Prison in the northeastern city of Hassakeh, the largest of around a dozen facilities run by US-backed Syrian Kurdish forces holding suspected Daesh fighters. Gweiran holds some 5,000, including Daesh commanders and figures considered among the most dangerous, according to Farhad Shami, a spokesman for the Kurdish-led Syrian Democratic Forces, or SDF.

The forces' commander, Mazloum Abadi, said Daesh mobilised "most of its sleeper cells” to organize the jailbreak.

Mourners gather in front of the casket of one of eleven Iraqi soldiers at a mosque in Najaf on Friday. AFP

The militants, armed with heavy machine guns and vehicles rigged with explosives, attacked Thursday evening, aiming to free their comrades, Shami said.

The fighting started with a large explosion around 7pm, followed by two more blasts later, said one resident whose home overlooks the area. The assault was complex. Prisoners inside the facility rioted and tried to break out simultaneously as a car bomb went off outside and gunmen clashed with security forces, Shami said. A car bomb hit a nearby petroleum depot, sparking a fire that still burned Friday.

Residents of the Ghweiran area in the prison's vicinity said intermittent fighting continued in parts of the neighbourhood, were some of the fleeing prisoners who had taken shelter remain.

Mourners carry the casket of one of eleven Iraqi soldiers through a street in the central holy shrine city of Najaf. AFP

The Kurdish-led group, which controls large swaths of northeastern Syria, denies any accusations of mistreatment of local Arabs and says it seeks to redress old grievances against them as a minority during decades of Arab nationalist rule.

The Daesh group claimed responsibility for the attack on its Aamaq news service Friday, saying it aimed to free prisoners and describing it as ongoing. Attempted prison breaks have been a main tactic of the group. During their 2014 surge in which they overwhelmed territory in Iraq and Syria, they carried out multiple prison breaks.

Friday’s attack in Iraq was a brazen strike on a barracks in the mountainous Al Azim district outside the town of Baqouba.