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.

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 rushing to the scene, killing a total of 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, Syria’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 embattled 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 20 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, noting that rebels have disrupted regime supply routes to the northern provinces of Aleppo and Idlib. “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 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 U.N. Security Council sanctions, while the rebels’ foreign backers have shied away from military intervention.

The U.S., 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-Qaida-inspired Jabhat al-Nusra, for example, is believed to be on the front lines in Aleppo and near Maaret al-Numan.

When Brahimi, the U.N.-Arab League envoy, first floated the idea of a holiday truce, he did not say what his long-term plan was. Even a temporary reduction in violence during such a truce would not have been a springboard for talks between Assad and the opposition on ending the war. Syria’s opposition says it will only negotiate if Assad resigns, a step the Syrian leader has refused to take.

Some said Brahimi’s initiative allowed a paralyzed international community to show briefly that it was doing something to try to end the war that has claimed more than 35,000 lives and displaced hundreds of thousands.

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 U.S. 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 violated key provisions, such as withdrawing troops from urban centers, and was widely held responsible for the collapse of the cease-fire.

Syrian Foreign Ministry spokesman Jihad Makdessi on Saturday accused the U.S. 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. The spokesman said Syria has sent messages to the U.N. Security Council concerning the violations.

Syrian state media accused the rebels of breaking the truce from the start.

One of the deadliest attacks Friday was a car bomb attack in a residential area of Damascus.

The state-run news agency SANA on Saturday quoted the director of Damascus Hospital, Dr. Adib Mahmoud, as saying the hospital received 15 dead civilians, including eight children, and 92 wounded, among them 65 children. Activists had put the death toll at 11.

Also Saturday, Lebanese broadcaster LBC TV said journalist Fidaa Itani, one of its employees covering Syria’s civil war, was detained by the rebels and is being held in the town of Azaz near the Turkish border.

The station quoted a local rebel leader in Azaz, Abu Ibrahim, as saying that rebels suspected Itani after he filmed many videos of rebels operations in Aleppo. Itani’s Lebanese cellphone was closed when The Associated Press tried to reach him.

The area also was the site of the May kidnapping of 11 Shiite Lebanese pilgrims who were on their way home from Iran. Two have been released while rebels say they will hold the others until Sheik Hassan Nasrallah, the leader of Lebanon’s militant Hezbollah group, apologizes to the Syrian people for supporting Assad.

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Associated Press writer Bassem Mroue in Beirut contributed to this report.