"Dozens of people were killed or wounded" in attacks that saw loyalist warplanes drop so-called barrel bombs near the Friday market in Aleppo city, the Syrian Observatory for Human Rights said.
Separate attacks also targeted the rebel-held Sakhur, Ahmadiyeh, Baideen and Ard al-Hamra neighbourhoods of the former commercial capital, it said.
The Aleppo Media Centre, a network of citizen journalists on the ground, also reported the attacks, adding that the barrel bomb attack "destroyed a bus, leaving no survivors".
It added that "hospitals are packed with wounded", and that the number of dead could not yet be confirmed.
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The air force was "continuing to bombard several areas of Aleppo," the AMC added.
It posted footage on the Internet showing pools of blood inside the wreckage of the bus that was hit.
A second video showed burning vehicles, while images distributed by activists showed the bloodstained faces of wounded children.
The Observatory also reported a man and his son were killed in bombing on the Aleppo province village of Atareb, while a man, woman and child from one family were killed in Marea in the same province.
The Syrian Revolution General Commission, a network of grassroots activists, described "panic and mass flight to the countryside, despite the intense cold".
Yesterday, Human Rights Watch accused regime forces of "wreaking disaster" in Aleppo with its intense aerial bombing campaign, which has killed hundreds in the past week.
In the central province of Homs, a car bombing today killed eight people, six of them schoolchildren, the official SANA news agency reported.
The Observatory reported a higher death toll of at least 12, including five children.
It said Omm al-Amd is home to a Shiite community close to the Alawite sect of President Bashar al-Assad, unlike the majority Sunni Muslim rebels fighting to topple him.