The jihadist militia "Islamic State" (IS) has attacked a prison in north-eastern Syria and may have freed a large number of inmates.

The Kurdish-dominated Syrian Democratic Forces (SDF) initially only confirmed the attack on Ghwajran prison in the city of Hassakeh on Thursday.

An unknown number of captured jihadists have been released, according to the UK-based Syrian Observatory for Human Rights.

According to the report, a car bomb exploded at the entrance to the prison and a second explosion occurred nearby before IS fighters attacked the Kurdish security forces head-on.

"A number of prisoners managed to escape," the observatory said, without giving any further details.

The observatory draws its information from a network of local whistleblowers.

The information provided by the organization can often hardly be checked by an independent party, but is considered reliable.

The SDF confirmed that there was an attempt to free IS fighters and that a car bomb was used.

Reinforcements were then sent to the prison and the area cordoned off.

There was initially no information about possible deaths or injuries.

According to the observatory, planes from the US-led international anti-IS coalition then flew over the area and dropped flares near the prison.

Ghwajran is one of the largest detention centers housing IS fighters in Syria, observatory director Rami Abdel Rahman told AFP.

IS was largely defeated militarily in Syria and Iraq in 2019.

Since their “caliphate” was crushed, the remaining fighters have either gone underground or retreated to more remote locations.