Syrian Revolution General Commission said the bomb attack which was carried out near Ghouta district of Damascus followed clashes between the Syrian regime forces and opposition groups.
Two soldiers royal to President Bashar al-Assad were killed prior to the attack, it said.
Syria has been gripped by almost constant fighting since the regime launched a violent crackdown in response to anti-government protests in March 2011, triggering a conflict, which has spiraled into a civil war.
The UN has stopped updating its death toll for the country because an official count had become too difficult to verify. At least 100,000 deaths were recorded in the last official count in July 2013.