An explosion at offices used by an ultraconservative militant group in northern Syria left at least seven people dead, opposition activists said.
The Local Coordination Committees said a car bomb blew up in close proximity to a building used by the Ahrar al-Sham group in the city of Aleppo on Monday, killing seven people and wounding others and damaging several buildings.
The Britain-based Syrian Observatory for Human Rights put the death toll at 23.
The opposition group, which relies on local activists, said that a suicide bomber driving a tanker truck was carried out the attack. The victims included four local commanders from Ahrar al-Sham, the Observatory said.
The Ahrar al-Sham group has been fighting against the Islamic State group for two years.
No one has claimed responsibility for the attack yet.