A devastating suicide truck bomb blast claimed by the so-called "Islamic State" killed nearly 50 people and wounded at least 150 in the northeastern Syrian city of Qamishli on Wednesday, the deadliest attack to hit the largely Kurdish city since the Syrian conflict began.

The attack targeted Kurdish security forces in a heavily guarded administrative part of the city. Qamishli acts as the de facto capital of the self-declared Kurdish autonomous region of Rojava.

The death toll was expected to rise due to the number of seriously injured. Emergency services, security forces and civilians were working to save people caught under the rubble, Kurdish media reported.

It was unclear how many of the dead and injured were security forces and civilians.

Kurdish forces, known as YPG, control broad swaths of territory in northeastern Syria along the border with Turkey, as well as pockets in Aleppo city and further to the northwest in Afrin.

The Syrian regime also remains in control of parts of Qamishli, including the intelligence headquarters and airport, as part of a tacit agreement with the Kurds. On occasion the two have engaged in light, short-lasting clashes.

Qamishli has been relatively quiet and avoided the type of destruction witnessed in other parts of Syria during the six year civil war. In April, a suicide blast killed six members of the Kurdish internal security forces.

The YPG, considered the best fighting force against IS, is the dominate force in the US-backed Syrian Defense Forces (SDF). Over the past year Kurdish forces have pushed back the extremist group on multiple fronts.

Brett McGurk, the US official leading the coalition fight against IS, condemned the attack and vowed to continue support to the Syrian Kurds.

In a statement on social media IS said it had carried out the attack in Qamishli in retaliation for an ongoing US-backed SDF offensive on Manbij.

The SDF have the strategic city completely surrounded. In recent weeks the SDF have sought to flush out IS militants from the city and free citizens under the terror group's grasp.

The capture of the city and surrounding countryside further to the north and west would be a major blow to IS, which relies on the route to move weapons and fighters between the Turkish border and its self-declared capital Raqqa.

cw/kms (AFP, Reuters)