The Islamic State group has claimed responsibility for the attack yesterday.
Reports said a truck loaded with large quantities of explosives blew up on the western edge of the town of Qamishli, followed by a second blast a few minutes later in the same area.
The blasts caused massive damage and rescue teams were working to recover victims from under the rubble, the Sana news agency said.
Though initially reported as a double bombing, it now appears that the first blast caused a petrol tank to explode.
Qamishli, near the Turkish border, is mainly controlled by Kurds but Syrian government forces are present and control the town’s airport.
A Kurdish-dominated militia, the Syrian Democratic Forces (SDF), backed by air strikes from the US-led coalition, is spearheading the battle against IS in northern Syria.
Syrian state TV broadcast footage showing people running away from a cloud of gray smoke rising over the town and others running amid wrecked or burning cars.
Qamishli resident Suleiman Youssef, a writer, said that he heard the first explosion from few miles away. He said the blasts levelled several buildings to the ground and many people were trapped under the rubble.
“Most of the buildings at the scene of the explosion have been heavily damaged because of the strength of the blast,” he said.
Hospitals in the city are calling for blood donations, the Syrian Observatory for Human Rights monitoring group has reported.
The dead include civilians and members of the security forces, it says.
While Qamishli has been targeted before, the group says this is the biggest explosion ever to hit the city.
The blast shattered windows in the Turkish town of Nusaybin, across the border.
IS said it carried out the attack in Qamishli, describing it as a truck bombing that struck a complex of Kurdish offices. The extremist group has carried out several bombings in Kurdish areas in Syria in the past.
The predominantly Kurdish US-backed Syria Democratic Forces have been the main force fighting IS in northern Syria, capturing significant territory from the extremists over the past two years.
Yesterday’s explosion came as US-backed Kurdish forces pressed ahead with their offensive to take the IS-held town of Manbij, also in northern Syria but further to the east of Qamishli.
Meanwhile the US is exploiting an enormous amount of digital information about the IS obtained by Syrian rebels who are fighting for control of the city of Manbij, a spokesman for the American-led military coalition said.
Speaking by phone from Baghdad, Colonel Christopher Garver told reporters at the Pentagon that it’s unclear how this trove of intelligence might affect the direction of the war, but he suggested it has been of considerable value.
“We think this is a big deal,” he said.
The intelligence is on laptop computers and portable data storage devices such as thumb drives. Garver said it amounts to more than four terabytes of digital information and sheds new light on how the IS has used Manbij as a “strategic hub” for welcoming, training, indoctrinating and dispatching foreign fighters.
Garver said the information has not provided links to those involved in attacks on Western targets directed or inspired by IS.