Syrian state media also reported that security forces in Damascus had arrested a second, would-be suicide bomber driving a car full of explosives near the site of the Mazraa bombing.
Damascus has so far mostly avoided the large-scale violence that has destroyed other Syrian cities, though deadly car bombings have targeted government buildings in the capital.
In May 2012, twin car bombs exploded outside a military intelligence building, killing 55 people in the deadliest attack against a regime target in the capital since the uprising began 23 months ago.
And in July, rebels detonated explosives inside a high-level crisis meeting in Damascus that killed four top regime officials, including Assad's brother-in-law and the defense minister.
Following that attack, rebel groups who have established footholds in suburbs of the capital pushed in, clashing with government forces for over a week before being routed and pushed out.
Since then, government jets have heavily bombed rebel-held suburbs and rebels have managed only small incursions on the city's south and east sides.
In Daraa, where Syria's uprising began nearly two years ago, the Observatory said the 18 people killed in the airstrike included eight rebel fighters, three medics, one woman and one young girl.
A video posted online said to be of the event showed the bodies of dead and wounded people being loaded in to the back of trucks and moved to another location. Some were bloody and had bandaged heads, while others were carried out on stretchers.
Syria's conflict began in March 2011 with political protests against the government and has since evolved into a civil war between Assad's regime and hundreds of rebel groups seeking to topple it. The U.N. says some 70,000 people have been killed in the conflict so far.
International diplomacy has failed to slow the fighting.