The explosion went off in the early afternoon outside a court house and security headquarters operated by opposition fighters who control the town of Azaz, resident Saif Alnajdi said.
“It hit the busiest part of the town,” Alnajdi said. Azaz, only a couple of miles away from the Turkish border, is a hub for anti-government activists and opposition fighters, as well as many people displaced by recent fighting in Aleppo. Activists say its pre-war population of 30,000 has swelled.
The town is sandwiched between rival groups, including Kurdish fighters to the west and opposition groups to the east, supported by Turkey. Islamic State militants have been pushed further east by the Turkey-backed fighters.
No one has yet claimed responsibility for the attack. Islamic State has been accused of carrying out attacks there before.
Activists shared photos of bodies lying in the street as large clouds of black smoke lingered overhead.
Rami Abdurrahman, the head of the Syrian Observatory for Human Rights monitoring group, said those killed included six opposition fighters. He said the explosion was caused by a rigged water or fuel tanker, which explains the large blast and high death toll. The activist-operated Azaz media centre put the death toll at 60, adding that search and rescue operations continued for hours after the explosion.
He said some of the severely wounded were transported across the border into the Turkish town of Kilis for treatment. The Turkish state-run Anadolu news agency said 53 wounded Syrians were brought to Kilis for treatment, including five in a critical condition, who were transferred to Gaziantep. The agency said one later died.
Alnajdi said some witnesses saw the vehicle drive into the town centre. The court house and the security headquarters were reportedly damaged, as well as the Red Crescent and municipality offices.
Footage shared online showed a large plume of black smoke rising above the chaotic street as onlookers gathered around the site.
Many rebels and civilians who were pushed out of Aleppo city during a massive government offensive late last year have resettled in Azaz.