The car bomb early Friday struck military offices in Sousian, which is behind the rebel lines about eight km (five miles) northwest of al-Bab, the Syrian Observatory for Human Rights, a Britain-based war monitor, said.
The Turkey-backed rebels on Thursday drove ISIL from al-Bab, the group's last significant stronghold in northwest Syria, along with two smaller neighbouring towns of Qabasin and al-Bezah after weeks of street fighting.