Two suicide bombers, one driving an ambulance packed with explosives, blasted an air force intelligence base near Damascus, a jihadist group which claimed the attack and a watchdog said on Tuesday.
The attack late Monday in Harasta just northeast of the Syrian capital sparked intense fighting between rebels and the army, which at daybreak pounded the town with shells, the Syrian Observatory for Human Rights said.
"We cannot confirm the exact number of casualties right now, but we expect the number is very high," Observatory director Rami Abdel Rahman told AFP.
The attack was claimed by the Al-Nusra Front, which was unknown before the start of the revolt against the regime of President Bashar al-Assad but which now regularly issues statements claiming suicide attacks in Syria.
"In revenge for those who have oppressed or killed Muslims, the decision was taken to strike the Air Force intelligence branch in Harasta," Al-Nusra said in a statement posted to online jihadist forums.
The group described a three phase operation in which 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 with a booby-trapped ambulance.
The fighters then targeted the area with mortars, according to the statement.
The Observatory confirmed that an air force intelligence branch in Harasta was bombed Monday night, followed by a second blast that hit a nearby army maintenance centre about 20 minutes later.
It said that the bombings shook the adjacent Damascus neighbourhoods of Qaboon and Jobar.
State media made no immediate mention of the bombings.