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.
On Thursday, British Foreign Secretary William Hague said his message to Assad is that ‘‘it is time to go.’’
He said the senseless killing must be brought to an end through a credible political process leading to a transition in Syria.
He also called on Assad to respond to a dialogue offer made recently by Syrian opposition chief Mouaz al-Khatib.
‘‘A political settlement, a political agreement on a transition is the way forward in Syria to bring to an end this terrible and unacceptable loss of life.’’
Al-Khatib has said he is open to talks with the regime that could pave the way for Assad’s departure, but that the Syrian leader must first release tens of thousands of detainees. The government has refused.
Hubbard reported from Beirut. Associated Press writer Zeina Karam contributed reporting from Beirut.