Rebels including Al-Qaeda's Syrian affiliate on Thursday launched a fresh offensive on the major southern city of Daraa, saying their aim was to "purify" the area, AFP reports.
Operation "Storm of Truth" was announced on Twitter by the Southern Front, an alliance of rebel groups, "to purify the province of Daraa from the filth of the gangs of (President Bashar Al-) Assad," the report said.
"The violent attack began this morning on regime-held parts of Daraa city, with both sides exchanging shellfire," Syrian Observatory for Human Rights director Rami Abdel Rahman told AFP.
"The groups are attacking government positions in northern parts of the city, but they haven't seized buildings yet," he added.
Al-Qaeda affiliate Al-Nusra Front and other Islamist militias are taking part in the fighting in Daraa, dubbed the "cradle of the revolution" against Assad which began in 2011 that triggered Syria's civil war.
An important rebel military commander has also died in the assault, the Observatory said.
Abou Hadi Abboud, who headed the Fallujah al-Houran opposition group, part of the Southern Front military coalition, was killed Thursday when a shell hit his car in Daraa province.
"He is the most important rebel commander to be killed since the beginning of the Daraa offensive in June," Abdel Rahman told AFP.
Regime aircraft have conducted at least 21 air raids and dropped 32 barrel bombs on rebel positions, according to the report.
Barrel bombs are crude weapons, made of containers packed with explosives and scrap metal, that are typically dropped from helicopters and have been routinely used by the Syrian regime.
Assad, however, has repeatedly denied using barrel bombs and has suggested in interviews that no such weaponry exists.
Late Thursday, a barrel bomb attack on the West Ghariya area of Daraa province killed 13 civilians, including two children.
"The two children were just reduced to body parts. From the waist down, there's nothing," Abdel Rahman said.
Another three children and a woman were killed in a rocket attack on a rebel-held neighborhood in Daraa city.
Opposition groups already control parts of the city and about 70 percent of the province of the same name, which borders Jordan.
Thursday's offensive appears to be a continuation of an assault in June, launched by the same groups but dubbed "Southern Storm." That round of fighting left at least 60 rebels, 18 loyalists and 11 civilians dead.
Syria's regime has already lost two provincial capitals -- Idlib in the northwest, which is held by a rebel alliance including Al-Nusra, and Raqa in the Euphrates valley, which is held by the Islamic State (ISIS) group.