A Syrian warplane flattened a three-storey building in Damascus as suspected rebels detonated a deadly car bomb in the country's east, further undermining a UN-backed truce on only its second day.
The unravelling of the ceasefire marks the latest setback in hopes to end Syria's civil war through diplomacy.
The proposed four-day truce during the Muslim holiday of Eid al Adha had been a long shot from the start, with international mediator Lakhdar Brahimi failing to get firm commitments from all combatants.
Fighting dropped off in the first hours of the ceasefire on Friday, but by the end of the day, activists said 151 people had been killed in bombings and shootings.
On Saturday, the first regime air strike since the start of the truce saw a three-storey building in the Damascus suburb of Arbeen reduced to rubble.
At least eight men died in the attack, according to the Britain-based Syrian Observatory for Human Rights.
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.
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 ceasefire.
The Syrian air force also bombed rebel positions during a fierce battle for control over the main road linking Aleppo, Syria's largest city and commercial hub, with the capital.
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 nightfall on Saturday, at least 49 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.
Mr Brahimi's spokesman declined to comment on the breakdown of the ceasefire.
It remains unclear what his next move could be, since the international community is divided over the Syria conflict that erupted 19 months ago.
The Syrian regime's allies Russia and China have shielded the country from harsher UN Security Council sanctions, while the rebels' foreign backers have shied away from military intervention.
The US, 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.
