BEIRUT — A Syrian warplane flattened a three-story building, suspected rebels detonated a deadly car bomb, and both sides traded gunfire in several hot spots across the country Saturday, activists said, leaving a UN-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 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.
On Saturday, the first regime airstrike since the start of the truce reduced a three-story building in the Arbeen suburb of the capital, Damascus to rubble, killing at least eight men, said the Britain-based Syrian Observatory for Human Rights, which compiles reports from activists.
In the remote eastern town of Deir el-Zour, assailants detonated a car bomb near a military police compound, then opened fire at those responding to the scene, killing eight people and causing extensive damage, the Observatory said. Syrian media denied there were casualties. The attack bore the hallmarks of Jabhat al-Nusra, a radical rebel-allied Islamic group that has rejected the cease-fire.
The Syrian air force also bombed rebel positions Saturday during a fierce battle for control over the main road linking Aleppo, the country’s largest city, with the capital, activists said. Earlier this month, rebels seized Maaret al-Numan, a town along the highway, and besieged a nearby military base, disrupting regime supplies to Aleppo. The Syrian air force has responded with sustained bombing raids on area villages.
By late Saturday, at least 76 people had been killed across Syria, including 16 Syrian soldiers, activists said. The Observatory reported deadly regime shelling and sniper attacks in several locations, while Syrian state-media said rebels ambushed a number of military positions.
Military analyst Joe Holliday said neither side has an incentive to halt fighting. ‘‘The regime can’t accept the current military status quo without a fight and the rebels have no reason to since they believe they have the momentum,’’ said Holliday, a researcher at the Institute for the Study of War in Washington.
Brahimi’s spokesman declined to comment Saturday on the apparent failure of his initiative. It’s not clear what Brahimi’s next move could be, since the international community is divided over the Syria conflict that erupted 19 months ago.
Assad allies Russia and China have shielded the regime against harsher United Nations Security Council sanctions, while the rebels’ foreign backers have shied away from military intervention.
The United States, meanwhile, is averse to sending strategic weapons to help the rebels break the battlefield stalemate, fearing they will fall into the hands of militant Islamists, who are increasingly active in rebel ranks. The Al Qaeda-inspired Jabhat al-Nusra, for example, is believed to be on the front lines in Aleppo and near Maaret al-Numan.
Shadi Hamid of the Brookings Doha Center said the truce at least ‘‘provides the illusion of movement, that something is being done, that the international community is still trying to find a solution.’’
The United States said Friday that both sides have violated the holiday cease-fire, but singled out the regime. In the previous attempted truce six months ago, the Syrian military was widely held responsible for the collapse of the cease-fire.
Syrian Foreign Ministry spokesman Jihad Makdessi on Saturday accused the US of being one-sided. He said Syria remains committed to halting military operations. He said all cease-fire violations were the result of attacks, most of them carried out by organizations that originally rejected the truce, an apparent reference to Jabhat al-Nusra.