The Britain-based Syrian Observatory for Human Rights says the attack in the village of Ziyara in the central province of Hama was carried out by Jabhat al-Nusra, an al-Qaida-inspired Islamic militant group.
The death toll could not be independently verified.
State-run news agency SANA said the blast occurred outside a state-run development agency. It said Monday's blast caused deaths but gave no figure.
The Observatory also reported that an air raid on the northern town of Harem killed at least 20 rebels. It said a rebel commander was seriously wounded in the raid.