A Syrian rides on the turret of a tank in the rebel-held area of Daraa on Feb. 4. (Mohamad Abazeed/AFP/Getty Images)

A cease-fire brokered by Turkey and Russia two months ago is fraying along Syria’s southern border as rebel forces launch their largest offensive in the area in more than a year.

Monitors, activists and an aid group said fighting between opposition and pro-government forces raged for a fourth day Wednesday in the southern city of Daraa, pushing the number of dead and wounded past 60.

A nationwide cease-fire has largely held since late December, when President Bashar al-Assad’s forces recaptured the flash-point northern city of Aleppo. That victory brought what remained of the armed opposition to a crisis point, bolstering the hand of its Turkish backers to negotiate a truce with one of the Syrian government’s staunchest allies, Russia.

But rebel guns had been largely silent in southern Syria for more than a year, constrained by infighting and the directives of the forces’ powerful Jordanian backers across the border.

In an unusual development for Syria’s knotty conflict, the rebel offensive in Daraa appeared to have been launched without international support. The fighters began pushing through the southwestern district of Manshiyah on Sunday, detonating car bombs and at least one powerful tunnel bomb. 

According to activists and the Syrian Observatory for Human Rights, a Britain-based monitoring network, the government has carried out intense airstrikes in response.

Medics said at least 25 people have been killed in the fighting, six of them from a single family. Thousands have fled the area, many seeking refuge on countryside farmland and in orchards.

The International Rescue Committee said a rocket attack late Monday injured four health workers and caused major damage to a clinic it supported in Daraa’s Balad district.

Amanda Catanzano, the organization’s senior director for international policy and advocacy, warned that similar attacks in Aleppo and other cities preceded heavy fighting. 

“It’s distressing to see the strategies that resulted in so many civilian deaths in Aleppo begin to play out elsewhere in Syria,” Catanzano said. “There can be no peace in Syria without accountability for these repeated attacks against civilians.”

But activists defended the fighting, insisting that the rebel offensive would bring a much-needed “morale boost” ahead of peace talks scheduled for Feb. 23 in Kazakhstan’s capital, Astana.

“This is a battle to bring spirit back to the opposition,” said Ahmed Almasalma, a media activist from Daraa’s western countryside.

Although an earlier round of talks in Astana ended inconclusively, negotiators hope that the new meetings will pave the way for a resumption of the Geneva peace process brokered by the United Nations.

Syria’s conflict, almost six years old, has gone through several cycles of cease-fires broken by ferocious violence. But the latest truce is playing out in a dramatically altered military and diplomatic landscape.

Rebel forces mostly have been boxed into the northwestern province of Idlib. Internal tensions have boiled over into open war there, with hard-line factions allied with al-Qaeda coming out on top. 

Pro-Assad forces hold all the most important urban centers in the country, and the president’s departure is no longer a precondition for the rebels’ participation in any peace process.

Zakaria Zakaria in Istanbul contributed to this report.