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. […]