The Syrian Observatory for Human Rights said at least 48 people, mostly civilians, were killed when the tanker blew up in front of a courthouse in the northern Syrian town of Azaz.
Around 14 of the dead were rebel fighters or courthouse guards, it said.
Dozens more had severe injuries, said the Observatory, which monitors the violence in the country.
In unsourced comments, Turkeys privately owned Dogan news agency said a car bomb planted by Islamic State was responsible.
There was no immediate claim of responsibility from the militant group.
Turkeys state-run Anadolu news agency cited a doctor in Azaz as saying at least 60 people had been killed and more than 50 wounded.
An Azaz resident who went to the local hospital told Reuters he had counted around 30 bodies laid out.
Syrias nearly six-year war has created a patchwork of areas of control across the country, and Azaz is a major stronghold of the Turkish-backed Free Syrian Army (FSA).
The FSA is an alliance of moderate rebel groups whose fighters have, with Turkish military support, pushed Islamic State militants out of the border area.
Saturdays explosion was heard across the border in the Turkish town of Kilis, Dogan said.
Anadolu said 23 wounded people had been taken by ambulance to Kilis hospital, one of whom subsequently died.
Southeast of Azaz, Turkish forces have launched an offensive to take back the Islamic State-held town of al-Bab.
In a round-up of its military operations in support of rebels in northern Syria, the Turkish military said that on Friday 21 Islamic State militants had been killed in clashes.
In a later statement, Turkeys army said that on Saturday one Turkish soldier was killed in clashes with the militants, while Turkish warplanes killed 37 militants, including three senior members of the group, in air strikes.
Editor: Priyambodo RH
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