At least 43 people were killed when a car bomb ripped through the centre of a busy commercial district of a rebel-held Syrian town along the Turkish border, witnesses have said.
The explosion went off early this afternoon outside a local courthouse and security headquarters operated by the opposition fighters who control Azaz, resident and activist Saif Alnajdi said.
"It hit the busiest part of the town," Mr Alnajdi said.
Azaz, only a few miles away from the Turkish border, is a hub for anti-government activists and opposition fighters, as well as many displaced from the recent fighting in Aleppo city.
Activists say its pre-war population of 30,000 has swelled. It is also sandwiched between rival groups, including Kurdish fighters to the west and Turkey-backed opposition groups to the east.
Islamic State militants have been pushed back farther east by the Turkey-backed fighters.
No-one has yet claimed responsibility for the attack. IS has been accused of carrying out attacks there before.
Footage of the aftermath of the explosion showed a gutted area. Activists shared photos of bodies lying on the pavements and in the middle of 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 local Azaz media centre put the death toll at 60, adding that search and rescue operations continued for hours after the explosion.
The observatory said the explosion took place near the local courthouse operated by rebel groups.
Mr Alnajdi said rescue workers were still working to identify those killed and ensure bodies were removed from the area, suggesting that the death toll was not final.
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' local hospital for treatment, including five in critical condition, transferred to Gaziantep. The agency said one later died.
Mr Alnajdi said some witnesses saw the vehicle - which he described as a tanker - drive into the town's centre.
The courthouse and the security headquarters were damaged, as well as the Red Crescent and municipality offices, according to activists in the area.
Media activist Baha al-Halabi, based in Aleppo province and who gathered information from people in Azaz, said witnesses reported many unidentified bodies because of the strength of the explosion.
Footage shared online showed a large plume of black smoke rising above the chaotic street and the sound of gunfire in the background as onlookers gathered around the site. In one instance, a father ran away from the scene, carrying his child to safety.