Opposition activists say an explosion at offices used by an ultraconservative militant group in northern Syria killed at least seven people.
The Local Coordination Committees said a car bomb exploded near a building used by the Ahrar al-Sham group in the city of Aleppo on Monday, killing seven people and wounding others.
The Britain-based Syrian Observatory for Human Rights put the death toll at 23.
The opposition group, which relies on local activists, said the attack was carried out by a suicide bomber driving a tanker truck. The Observatory says the dead include four local Ahrar al-Sham commanders.
The Turkey and Saudi-backed Ahrar al-Sham has been fighting against the Islamic State group for two years.
No one claimed responsibility for the attack, which damaged several buildings.