Three children were among at least 11 civilians killed on Wednesday when a Syrian government aircraft dropped a barrel bomb on a rebel-held neighbourhood of Aleppo, a monitoring group said.
The air raid came a day after rebel mortar fire on government-held areas of the northern city killed 38 people, including at least 18 children, drawing UN condemnation.
The crude explosives-filled container caused four buildings to collapse on residents in the rebel-held Saad al-Ansari neighbourhood, the Syrian Observatory for Human Rights said.
"Among the dozens wounded, some lost limbs," Observatory chief Rami Abdel Rahman told AFP.
Nine people, including at least two children, were pulled from the rubble after being trapped for five hours.
"The rescue operation was delayed because it's a working-class neighbourhood with narrow streets and there was a power cut," Abdel Rahman said.
Once Syria's economic powerhouse, Aleppo has been ravaged by fighting since the rebels seized the east of the city in 2012, confining government forces to the west.
Thousands of civilians have been killed in government air strikes on rebel-held districts, while hundreds more have been killed by rebel rocket or mortar fire on government-held areas.
Tuesday's death toll from rebel fire was one of the highest of the war so far, the Observatory said, and it drew condemnation from the UN children's agency.
"These attacks highlight a flagrant disregard for the laws of war," said UNICEF Syria representative Hanna Singer. "They are a stark reminder that nowhere in Syria is safe for children."
More than 240,000 people have been killed in Syria since the conflict began with anti-government protests in March 2011.