It was not immediately clear who was behind the attack, the BBC reported. Local security forces were focusing their investigations on the likely involvement of the Kurdish People’s Protection Units (YPG) and Kurdistan Worker’s Party (PKK), Turkey’s state-run Anadolu news agency said.

Members of the rescue organization White Helmets were on the ground, searching for survivors and the wounded have been taken to nearby hospitals. The car bomb was the second attack in a week, with the previous one killing two people and wounding seven others at a security checkpoint, according to Anadolu.

Turkish-backed Syrian groups have controlled al-Bab since Turkey’s 2016 Operation Euphrates Shield against the Kurdish-dominated Syrian Democratic Forces (SDF).

The YPG is a group that serves as a military wing of the majority Kurdish multi-ethnic SDF, which was the major force on the ground in the U.S.-led international coalition against the Islamic State (ISIS). Turkey considers the YPG to be an offshoot of the PKK, while its Western allies do not recognize it as so.

Turkey has carried out three large scale operations in northeast Syria since 2016, mostly targeting Kurdish militants. It may be preparing another military incursion. Last week, Turkish President Recep Tayyip Erdogan said the country would go in to “clear terrorist zones that still exist in Syria” if Russia and the United States did not do so themselves.

Reporter’s code: 50101