A fuel truck has exploded in the centre of a rebel-held town near Syria’s border with Turkey, killing dozens of people.

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 town of Azaz. Around 14 of the dead were rebel fighters or courthouse guards, it said. Dozens more were injured.

Turkey’s 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.

Osama al-Merhi, a lawyer at the scene of the blast, also pointed the finger at IS.

“These kinds of crimes are only committed by the terrorist group Daesh,” he told AFP, using an Arabic acronym for IS. “They are the ones who target civilians and the cadres who are building this country.”

Turkey’s state-run Anadolu news agency cited a doctor in Azaz as saying that 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.

The attack appeared to be the deadliest yet in the town in northern Aleppo province, which has been regularly hit by bombings targeting rebels and civilians. In November rebels said 25 people – civilians and opposition fighters – were killed in a car bombing of a rebel headquarters. The rebels accused the Islamic State group of being behind that attack.

Syria’s nearly six-year war has create a patchwork of areas of control across the country.Azaz is a stronghold of the Turkish-backed Free Syrian Army (FSA), an alliance of moderate rebel groups whose fighters have, with Turkish military support, pushed Islamic State militants out of the border area.

In a roundup of its military operations over the last 24 hours in support of rebels in northern Syria, the Turkish military said on Saturday that 21 Islamic State militants had been killed in clashes.

It said its warplanes had destroyed buildings and vehicles in airstrikes on 12 Islamic State targets.

The blast comes during a fragile nationwide ceasefire brokered by Syrian government ally Russia and Turkey. The truce came into effect on December 30 and is intended to pave the way for new peace talks in Kazakh capital Astana, which regime ally Iran is also helping organise.

But the ceasefire and the planned talks have been threatened by ongoing violence in the rebel-held Wadi Barada region outside Damascus, which is the main water source for the capital.