Much of Daraa has been reduced to rubble and the two sides are exchanging fire through the shells of buildings, according to footage released by Syrian military media and the al-Qaida-linked Abaa News Agency over the weekend.
Local opposition activist Ahmad al-Masalmeh recorded 90 missile and bomb strikes by government forces by the afternoon. Syrian military media footage showed clouds of smoke and dust rising over the city.
At least 31 fighters have been killed in the clashes since late Friday, with fatalities distributed nearly evenly between the two sides, according to the Britain-based Syrian Observatory for Human Rights monitoring group.
The fighting has underscored the frailty of the "de-escalation" agreement brokered by Russia, Turkey, and Iran one month ago. The three powers are supposed to guarantee cease-fires across four zones in Syria, including Daraa. Russia said it would delineate the four zones by Sunday, but no announcement has yet been made.
Earlier in the weekend, rebels, including al-Qaida-linked fighters, attacked government and allied positions in the city's Manshiyeh district. "The regime sent 200 of its men to Manshiyeh yesterday. The fighters were afraid of an operation, so they attacked first," al-Masalmeh told The Associated Press.
Casualties could not be independently verified, but a rebel operations room said several Hezbollah fighters had been killed. The Lebanese militant group has deployed thousands of its fighters to Syria in support of President Bashar Assad's forces.
The Observatory said at least three different parts of the city came under rocket and missile attack by government forces on Sunday, and that rebels returned fire. Syrian military media distributed footage showing its planes and artillery striking built up areas.
The government and its allies have sent reinforcements to the city, including at least seven tanks and 20 other armed vehicles, according to al-Masalmeh, who said rebels could see the government's troop movements on the highway connecting Daraa to Damascus.
Daraa saw some of the earliest demonstrations against the Assad family's four-decade rule in 2011. A violent government crackdown and the rise of an armed insurgency eventually plunged the country into civil war.
The fighting has displaced half of Syria's population and claimed some 400,000 lives. Daraa's population was around 100,000 before the war. Elsewhere in Syria, the U.S.-backed Syrian Democratic Forces reported capturing a third dam from Islamic State militants on the Euphrates River in the north on Sunday as they approached Raqqa, the militants' de facto capital. A spokeswoman for the SDF said Saturday the group would launch its battle for Raqqa "within days."
The Kurdish-led group aims to establish an autonomous zone along the Turkish border.