On Wednesday, Syrian Kurds, supported by the United States, announced that they had retaken control of a major prison that Islamic State extremists attacked on January 20 in the city of Hassakeh in northeastern Syria.

The jailbreak triggered fierce clashes between both sides.

At least 332 people have since been killed in the violence, the Syrian Observatory for Human Rights said on Sunday.

The dead included 246 Islamic State elements, 79 Kurdish security personnel and seven civilians, the Britain-based watchdog said.

The toll could further rise as dozens of people were seriously injured in the violence and others were still unaccounted for.

A mutiny by Islamic State militants inside the prison is over, but combing operations inside and outside the site continue, the observatory's head Rami Abdel-Rahman said.

"The United States commends the Syrian Democratic Forces (SDF), who, along with Coalition forces, completed operations to re-take full control of the Hasakah prison in Syria," US National Security Advisor Jake Sullivan said in a statement.

"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."

The raid by extremist militia Organisation Islamic State on the prison run by the Kurdish-led SDF was a daring operation aimed at freeing its inmates.

The attack was the fiercest by Islamic State in Syria in almost three years, the observatory said.

The attack has fuelled fears of an Islamic State resurgence in war-torn Syria.

Hassakeh, controlled by Syrian Kurds, is located in the northeast of Syria.

According to Kurdish media, around 5000 Islamic State supporters were most recently locked up in the prison.

Although the terrorist militia has been defeated militarily in Syria, sleeper cells remain active and carry out occasional attacks.

Australian Associated Press