Syrian warplanes bombed the old district of Deraa city on Thursday for the first time in the country's 23-month-old revolt, killing 18 people, activists said.
A rebel officer in the Tawheed al-Janoub brigade which led a rebel offensive this week in Deraa, cradle of the uprising which erupted against President Bashar al-Assad in March 2011, said there were at least five air strikes on the city on Thursday.
"The (rebel) attacks on several major checkpoints in the Hay al-Saad neighbourhood and its declaration as a liberated area has prompted this response," said Abdullah Masalmah, an activist from the city, reached on Skype.
This week's rebel offensive destroyed several army fortifications, rebel sources said.
The Syrian Observatory for Human Rights, a British-based group which monitors a network of sources inside Syria, put the death toll at 18, including eight rebel fighters.
Fighting has intensified in southern Syria in the last few weeks, leading to a sharp increase in refugee flows to neighbouring Jordan, according to officials. A Jordanian military source said 4,288 refugees arrived in the last 24 hours alone.