"The timing of the attack is significant," coming days after troops loyal to President Bashar al-Assad recaptured several areas that had been under rebel control for a year, he told AFP.
The explosives appeared to have been placed "either inside or beneath the building in a tunnel", Abdel Rahman added, suggesting rebels may have infiltrated the base.
A rebel group, the Direh al-Aasmeh (Shield of Damascus) brigade, claimed responsibility for the attack.
A video distributed by the group, which is part of the Western-backed Free Syrian Army, showed the building collapse completely.
North of the capital, Syria's air force launched air raids on Qara near the border with Lebanon as loyalist forces tried to storm rebel positions in the town, the Observatory said.
"Since the morning, the town of Qara has been hit by air strikes," said the Observatory's Abdel Rahman.
"Warplanes bombarded the town heavily yesterday (Saturday). Regime troops are trying to storm it and to drive the rebels out."
The Britain-based group said opposition fighters in Qara appeared determined to resist despite the onslaught.
Pro-regime newspaper Al-Watan reported that "the army hit the Qalamoun mountains hard, closing in on the terrorists around Qara," using the government's term for rebels.
Violence in the Qalamoun area intensified on Friday.
Both the regime, backed by fighters from powerful Lebanese Shiite movement Hezbollah, and rebels including jihadists affiliated to Al-Qaeda have bolstered their forces in the area.
Qalamoun, which has a strong rebel presence, is strategic because it borders Lebanon and is used as a rear base for operations around the capital.
For Assad's regime, it is important because it is on the road linking Damascus to the central province of Homs and also houses weapons depots.
Battered by shelling
For months, Qalamoun mostly avoided the violence tearing other areas of Syria apart, but in past weeks parts of the town have been battered by near-daily shelling.
Lebanese authorities said 500 families fleeing Syria arrived in the border town of Arsal, in eastern Lebanon, on Sunday.
The latest influx brought the number of families who arrived in Arsal this weekend alone to 1,700, said Lebanon's social affairs ministry. Most had fled from Qara.
Arsal's municipality chief Ali al-Hojairi told AFP: "Many are sleeping in cars. They need shelter."
More than 800,000 Syrians fleeing the civil war have taken refuge in Lebanon, and local authorities and international agencies are struggling to provide for them.
Elsewhere in Syria Sunday, Damascus and several parts of the south were hit by a power cut lasting several hours.
Electricity Minister Imad Khamis called it "the result of sabotage by armed terrorist groups against the high voltage cables that feed the southern areas."
Anti-regime activists blamed regime bombing of the Qalamoun area for the outage.
Power cuts have become regular in Syria as the armed conflict that started in March 2011 as a rebellion against the Assad regime has intensified.
Also on Sunday, mortar rounds slammed into areas of central Damascus, killing at least two people, said the Observatory.
The escalation of violence in and around Damascus comes as loyalists advancing towards Aleppo.
On Sunday, rebels shot down a warplane over the east of the city, activists reported.
In a separate development, Turkish soldiers shot dead thee Syrians attempting to cross the border illegally, Turkish news agency Dogon reported.
More than 120,000 people have been killed in Syria's brutal war, and millions more forced to flee their homes.
On the political front, opposition chief Ahmed Jarba is "very interested" in travelling to meet Russian officials in Moscow, a major backer of the Damascus regime, a Jarba adviser told AFP.