A surge in fighting across Syria on Thursday signaled the apparent collapse of a landmark cease-fire that has been under mounting stress in recent days because of intensifying assaults by government forces and rebels.

The partial truce, which took effect in late February, represented a rare moment of agreement over the Syrian conflict between its most powerful outside players: Russia and the United States.

Although Moscow backs Syrian President Bashar al-Assad and Washington supports his opposition, the powers cajoled their Syrian allies into an agreement to cease hostilities to promote peace talks in Geneva that resumed Wednesday. The burst of fighting will almost certainly complicate those talks — now in their second round — and prolong a civil war that has killed more than 250,000 people and displaced millions. […]