Fighting raged for a third day Saturday between the Islamic State group and Kurdish forces in Syria after IS attacked a prison housing jihadists, with the violence killing nearly 90 people, a monitor said.

The assault on Ghwayran prison in the northeastern city of Hasakeh is one of IS's most significant since its "caliphate" was declared defeated in Syria nearly three years ago.

"At least 28 members of the Kurdish security forces, five civilians and 56 members of IS have been killed" in the violence, said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.

IS launched the attack late Thursday against the prison, which houses at least 3,500 suspected members of the jihadist group, including some of its leaders, the Observatory said.

The jihadists seized weapons and freed several fellow IS fighters, said the Britain-based monitor, which relies on sources inside war-torn Syria.

Map locating Hassakeh in Syria Map locating Hassakeh in Syria Photo: AFP / STAFF

Hundreds of jihadist inmates had since been recaptured but dozens were still believed to be on the run, it added.

With air support from the US-led coalition, Kurdish security forces have encircled the prison and are battling to control surrounding neighbourhoods, which jihadists have used as a launchpad for their attacks.

The Kurdish-dominated Syrian Democratic Forces (SDF) said "fierce clashes" broke out in neighbourhoods north of Ghwayran, where it carried out raids and killed more than 20 IS fighters.

An AFP correspondent saw Kurdish fighters raiding homes near the jail in search of militants while coalition helicopters flew overhead.

In one location, Kurdish fighters gathered around five bloodied corpses of suspected IS fighters along a roadside, the correspondent said.

The United States condemned the IS attack, praised the SDF's response and noted that IS fighters have been trying to free jihadist prisoners for more than a year.

A file picture shows a member of the Syrian Democratic Forces (SDF) standing guard in a prison where suspected Islamic State group jihadists are jailed in Hasakeh A file picture shows a member of the Syrian Democratic Forces (SDF) standing guard in a prison where suspected Islamic State group jihadists are jailed in Hasakeh Photo: AFP / FADEL SENNA

"It is a credit to the diligent, capable efforts of the SDF and Global Coalition to Defeat ISIS Forces that they were able to thwart numerous attacks during that period and limit the severity of this one," State Department spokesman Ned Price said in a statement, using another acronym for IS.

The battles have triggered a civilian exodus from neighbourhoods around Ghwayran, with families fleeing in the harsh winter cold as Kurdish forces closed in.

"Thousands have left their homes near the prison, fleeing to nearby areas where their relatives live," Sheikhmous Ahmed, an official in the autonomous Kurdish administration, told AFP.

Syrians flee their homes in the Ghwayran neighbourhood of the northern city of Hasakeh on the third day of fighting between Islamic State group fighters and Kurdish forces after IS attacked a prison housing jihadists Syrians flee their homes in the Ghwayran neighbourhood of the northern city of Hasakeh on the third day of fighting between Islamic State group fighters and Kurdish forces after IS attacked a prison housing jihadists Photo: AFP

Not all the displaced found safe haven.

"We don't know where we are going," said Abu Anas, who was forced out of his home on Saturday.

"We have no one but God," he told AFP as he fled on foot with his wife and four children.

IS has carried out persistent attacks against Kurdish and government targets in Syria since the rump of its once-sprawling proto-state was overrun in March 2019.

Most of them have targeted remote military outposts and oil installations, but the Hasakeh attack could mark a new phase in the group's resurgence.

An IS video released Saturday by the group's Amaq news agency purported to show armed jihadists infiltrating the prison at the start of the operation.

They raised the group's black flag as they stormed the facility and surrounded what appeared to be a group of prison guards.

AFP could not independently verify the authenticity of the footage.

It was not immediately clear whether the prison break was part of a centrally coordinated operation -- timed to coincide with an attack on a military base in neighbouring Iraq -- or the action of a local IS cell.

Analyst Nicholas Heras of the Newlines Institute in Washington said the jihadist group targeted the prison to bolster its numbers and "move beyond being the terrorist and criminal network that it has devolved into."

"Prison breaks represent the best opportunity for ISIS to regain its strength in arms, and Ghwayran prison is a nice fat target for ISIS because it's overcrowded," he told AFP.

The Kurdish authorities have long warned they do not have the capacity to hold, let alone put on trial, the thousands of IS fighters captured in years of operations.

Kurdish-run prisons now hold more than 12,000 IS suspects representing more than 50 nationalities, they say.

Many of the prisoners' countries of origins have been reluctant to repatriate them.

The war in Syria, which broke out in 2011, has killed close to half a million people and spurred the largest conflict-induced displacement since World War II.