As the violence raged, Britain accused the Syrian regime of effectively murdering a British surgeon it had jailed after seizing him in Aleppo, where he had volunteered to work at a hospital.
Two children were among at least 20 people killed in Aleppo yesterday, according to the Syrian Observatory for Human Rights, a Britain-based group that relies on activists and other witnesses in Syria.
Air strikes on rebel-held areas Sunday and Monday killed 86 people, including 32 children, the Observatory said.
An estimated 126,000 people have been killed in Syria's civil war, which erupted after a brutal crackdown on pro-democracy protests first held in March 2011.
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International humanitarian organisation Doctors Without Borders (MSF) corroborated reports that regime helicopters started dropping barrel bombs on Aleppo on Sunday.
In just two cases -- the targeting of a school and a roundabout where people wait for public transport -- "there were dozens of dead and injured people," MSF Syria coordinator Aitor Zabalgogeazkoa said.
"The hospitals in the area are overrun and are asking for medical supplies. We sent them immediately," Zabalgogeazkoa said.
Earlier, the Observatory and activists said the air force had been dropping barrels of explosives from both helicopters and fixed-wing aircraft.
Observatory director Rami Abdel Rahman said there was a "marked escalation" in attacks, and that "this type of intensive bombing over several days demonstrates the desire of the army to advance" on rebel-held areas.
The opposition National Coalition said the "systematic raids on Aleppo demonstrate the regime's rejection of a political solution" to the war.
The violence in Syria appeared to spill over into neighbouring Lebanon again yesterday, when a car exploded and caused an unknown number of casualties near a Hezbollah post in the east of the country.
In a separate incident, several rockets rained down on the eastern Lebanese town of Hermel, a Hezbollah bastion, with one hitting an army barracks and wounding two soldiers, the Lebanese military said.
Hezbollah, a Lebanese Shiite movement, has been the target of repeated attacks by suspected Sunni hardliners since its leader, Hassan Nasrallah, confirmed in April that its fighters were in Syria supporting Assad's forces.