BEIRUT – A Syrian warplane flattened a three-story building, suspected rebels detonated a deadly car bomb and both sides traded gunfire in several hotspots across the country Saturday, activists said, leaving a U.N.-backed holiday truce in tatters on its second day.

The unraveling of the cease-fire marked the latest setback to ending Syria's civil war through diplomacy. Foreign military intervention is unlikely, raising the grim prospect of a drawn-out war of attrition between President Bashar Assad and those trying to topple him.

The proposed four-day truce during the Muslim holiday of Eid al-Adha had been a long shot from the start since international mediator Lakhdar Brahimi failed to get solid commitments from all combatants. Fighting dropped off in the first hours of the cease-fire Friday, but by the end of the day, activists said 151 people had been killed in bombings and shootings, a standard daily toll in Syria.