The strikes come after Russia and the Syrian regime spurned a US plea to halt flights, burying any hopes for the revival of a ceasefire declared earlier in September.
In the heaviest attack in months, fighter jets pounded the opposition-held districts of the Syrian city of Aleppo overnight, days after a week-long ceasefire brokered by the US and Russia collapsed.
Hamza al-Khatib, the director of a hospital in the rebel-held east, told Reuters the death toll from the strikes was 45.
The strikes come after Russia and the Syrian regime rejected a US plea to halt flights, burying any hope for the revival of the ceasefire.
Opposition officials and rescue workers said incendiary bombs were among the weapons used on the city.
"It's as if the planes are trying to compensate for all the days they didn't drop bombs during the ceasefire," Ammar al-Selmo, the head of the Civil Defense rescue service in opposition-held eastern Aleppo, told Reuters.
"It was like there was coordination between the planes and the artillery shelling, because the shells were hitting the same locations that the planes hit," he said.
The assault by aircraft from the Syrian regime, its Russian allies, or both, made it clear that Moscow and Damascus had rejected a plea by US Secretary of State John Kerry to halt flights so that aid could be delivered and a ceasefire salvaged.
There was no immediate comment from the Syrian regime or mention on regime-run media of Thursday's bombardment.
"It was the heaviest air strikes for months inside Aleppo city," said Rami Abdulrahman, director of the Syrian Observatory for Human Rights, which monitors the conflict from Britain.
Assad has appeared as uncompromising as ever in recent weeks, reiterating his goal of taking back the whole country on the day the US-Russian brokered truce took effect.
The regime's main focus has been to consolidate its grip over the main cities of western Syria and the coastal region that is the ancestral homeland of Assad's Alawite sect.
On Thursday, around 120 opposition fighters and their families were evacuated from the last opposition-held district of Homs under an agreement with the regime by which they were given safe passage to nearby opposition-held areas.
The opposition says such agreements are part of a regime strategy to forcibly displace populations from opposition-held areas after years of siege and bombardment.
The Syrian Observatory for Human Rights estimates several thousand opposition fighters remain in the al-Waer district.
UN resumes aid deliveries
A "clearly marked" UN convoy was due to deliver aid on Thursday to besieged areas near the Syrian capital after a 48-hour suspension to review security guarantees following an attack on relief trucks near Aleppo, a UN spokesman said.
The UN suspended land deliveries after the convoy attack, which the Syrian Arab Red Crescent says killed a staff member and around 20 civilians.
US officials believe Russian aircraft were responsible for the strike, but Moscow has denied involvement and the Russian Defense Ministry said on Wednesday that a US Predator drone was in the area when the convoy was attacked.
"We are sending today an inter-agency convoy that will cross conflict lines into a besieged area of rural Damascus," Jens Laerke, spokesman for the UN Office for the Coordination of Humanitarian Affairs (OCHA), told Reuters.
"We will advise on the exact locations once the convoy has actually reached those locations."
Assad says Syria war will "drag on"
Syrian regime leader Bashar al-Assad said in an interview with AP News broadcast on Thursday that Syria's war will "drag on" as long as it is funded and interfered in by other countries.
"When you talk about it as part of a global conflict and a regional conflict, when you have many external factors that you don't control, it's going to drag on," Assad said.
Assad told AP that Russia was not behind a deadly attack on an aid convoy on Monday, which US officials have said they believe Moscow was responsible for.
Assad cast doubt on the intentions of the United States in Syria, saying it "doesn't have the will" to fight militants.
"I don't believe the United States will be ready to join Russia in fighting terrorists in Syria...the United States is not genuine regarding having a cessation of violence in Syria."