BEIRUT: Seven Hezbollah fighters have been killed in clashes with Syrian rebel forces in the country’s south in recent days, a Lebanese security source said Sunday, as government forces pounded parts of the city of Deraa with airstrikes and artillery fire.

The source told The Daily Star that the deaths were mainly concentrated around Deraa, where rebels launched an attack on government positions Thursday.

A rebel operations room also said several Hezbollah fighters had been killed, without specifying numbers. Hezbollah has deployed thousands of its fighters to Syria in support of President Bashar Assad’s forces.

Much of Deraa 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-Qaeda-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 Syrian Observatory for Human Rights activist group.

The fighting has underscored the frailty of the “de-escalation” accord brokered by Russia, Turkey, and Iran one month ago. The three powers are supposed to guarantee cease-fires across four zones in Syria, including Deraa. Russia said it would delineate the four zones by Sunday, but no announcement has yet been made.

Earlier in the weekend, rebels, including Al-Qaeda-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,” Masalmeh told the Associated Press.

The Observatory said at least three different parts of the city came under rocket and missile attack by government forces 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 Masalmeh, who said rebels could see the government’s troop movements on the highway connecting Deraa to Damascus.

Deraa saw some of the earliest demonstrations against the Assad family’s four-decade rule in 2011.

The fighting has displaced half of Syria’s population and claimed some 400,000 lives.

Deraa’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 Daesh (ISIS) militants on the Euphrates River in the north 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.”

Also Sunday, Syria’s army seized a key town from Daesh in the east of Aleppo province, a military source said, nearing its goal of expelling the militants from the region.

The capture of Maskana, on the western bank of Lake Assad, comes as part of a major Russian-backed military operation that began in mid-January to drive the militants from Aleppo province. “Security and stability has been restored to the strategic town of Maskana and a number of areas,” military source cited by Syrian state media said. Maskana was a key target for the Syrian army, and lies around 15 kilometers from neighboring Raqqa province.

A military source told AFP the “Maskana area is the last important civilian population center on the eastern border of Aleppo province before Raqqa province.”

“Whoever controls Maskana controls the axes running between Aleppo city and Raqqa city,” he added.