More than 40 people have been killed and dozens injured in a car bomb blast in a rebel-held Syrian town 7 kilometres from the Turkish border, activists and rescue workers said.

The explosion happened outside a local courthouse and security headquarters in the town of Azaz in the northern province of Aleppo, damaging a government building and a local courthouse.

Video and photos purporting to be of the aftermath of the blast showed a panicked scene with dozens of injured people and charred burnt bodies.

The Syrian Observatory for Human Rights said six rebels were among the dead, with the rest believed to be civilians.

Other activist groups, including the Azaz Media Centre, put the death toll at 50, adding that search and rescue operations continued for at least two hours after the explosion.

Some local media reported that as many as 60 people may have been killed.

An Azaz resident who went to the local hospital told the Reuters news agency he had counted about 30 bodies laid out.

The town is a major stronghold of the Turkish-backed Free Syrian Army.

With Turkish military support, its fighters had pushed Islamic State (IS) militants out of the border area near Azaz.

It is the heaviest casualty toll in a single incident since the latest nation-wide ceasefire in Syria started last week.

No group has yet claimed responsibility for the bombing but Azaz has recently been targeted by IS.

Azaz home to many rebels, civilians from Aleppo

The Observatory said the explosion took place near the local courthouse operated by rebel groups.

Media activist Baha al-Halabi, based in Aleppo province and who gathered information from people in Azaz, said witnesses described seeing many unidentified bodies because of the strength of the explosion.

Images shared online showed a huge plume of black smoke rising above the chaotic market and sounds of gunfire echoing in the background as onlookers gathered around the site.

Azaz is near the Syrian border with Turkey.

Bodies were strewn on the ground as a father ran away from the scene hugging his child.

Many rebels and civilians who were pushed out of Aleppo city during a massive government offensive late last year resettled in Azaz.

To the west of Azaz, Syrian Kurdish forces have control of a swath of land, and they have often tried to advance toward the town, causing friction with Turkish troops and allied Syrian opposition fighters.

To the east, opposition fighters backed by Turkey have been pushing back Islamic State extremists, gaining territory and advancing on the IS-stronghold town of al-Bab, further east.