The attack happened happened in Harasta, just northeast of the Syrian capital Twin blasts have hit a military base near Damascus by suicide bombers, one driving a bomb-laden ambulance, killing dozens of people. The attack, the latest in a spate of assaults on Syrian military and government installations, was claimed by the Al-Qaeda-linked Al-Nusra Front, which said it was to avenge Muslims "oppressed or killed" by the regime.
The Al-Nusra Front was unknown before the start of the revolt against Assad's regime but now regularly issues statements claiming suicide attacks in Syria. The group said it was behind the Harasta attack.
Describing a three-phase operation, a suicide bomber drove a car loaded with nine tonnes of explosives to the front of the building and 25 minutes later, another fighter drove through in a booby-trapped ambulance.
The militants then targeted the area with mortars, according to the statement.
The attack sparked intense fighting in Harasta between rebels and the army, which at daybreak pounded the town with shells, the Britain-based Observatory said.
It said Syrian forces on Tuesday also rained shells down on rebel belts in the second city of Aleppo, which has been fiercely contested since mid-July, and in Idlib province near the Turkish border.
News of the blasts came as UN chief Ban Ki-moon urged President Bashar al-Assad's regime to declare a unilateral truce in the almost 19 month conflict which activists say has killed more than 32,000 people.