The crude, unguided explosive device hit the Qaterji neighbourhood, where an AFP photographer saw a street strewn with rubble as residents ran for safety and a rescuer rushed a bloodied child into an ambulance.
The Syrian Observatory for Human Rights said nine civilians were killed in Qaterji and two others, including a child, were killed in the Mayssar neighbourhood of the northern city.
Another five civilians were killed in two other districts and on the city's outskirts.
Further down the street two women and two children scrambled for safety past the mangled iron shutters of shops and buildings badly damaged by the barrel bomb, the AFP photographer said.
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The rescuer, his hair covered in white dust, carried a child with a blood-covered face to an ambulance in which another wounded child already lay.
A truce agreed by Russia and the United States in February has been violated nearly continuously around Aleppo, where the regime and rebel groups have fought for control since 2012.
More than 300 civilians have been killed in Aleppo since April as rebels have pounded government-controlled neighbourhoods with rocket and artillery fire and the regime has hit rebel areas with air raids.
At least 280,000 people have been killed and millions displaced since Syria's war started in 2011 with the brutal repression of anti-government protests.