A rebel bomb attack on an army base near Damascus killed 31 Syrian soldiers on Sunday, AFP reports.
The blast came amid a major regime offensive against rebel positions all around the capital and on the city of Aleppo in the north, the news agency said.
"Three generals and a brigadier-general were among 31 troops killed in a bomb attack that caused a building in the army transport base in Harasta to collapse," according to Syrian Observatory for Human Rights director Rami Abdel Rahman.
The explosives appeared to have been placed "either inside or beneath the building in a tunnel", Abdel Rahman told AFP, 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 collapsing completely.
Other footage which emerged on Sunday appeared to illustrate the rebels' increasing success in targeting the Syrian air force.
The videos are mostly from the area of Aleppo, where fierce fighting has been raging as pro-regime forces seek to take back control of Syria's second city.
Much of the footage demonstrates how rebels are making up for their lack of sophisticated anti-aircraft weaponry by targeting military aircraft on the ground, using Chinese-developed HJ-8 anti-tank missiles, which are portable and relatively easy to deploy.
Most of their successes appear to be against government helicopters, although several clips - some of which cannot be verified - purport to show rebels successfully downing air force jets as well.
Meanwhile on Sunday, 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 told AFP.
The Britain-based group said opposition fighters in Qara appeared determined to resist despite the onslaught.
The attacks appear to be part of a long-awaited pro-regime offensive on the strategically-important Qalamoun region, aimed at cutting off rebel supply-lines with Lebanon.
The UN says more than 100,000 people have been killed since the uprising against President Bashar Al-Assad began in 2011.