Government forces, meanwhile, dropped crude "barrel bombs" in deadly airstrikes as both sides escalated their fight for the strategic city ahead of a second round of peace talks set for next week.
Opposition leaders threatened to suspend the talks over the barrel bombings.
In the past six days alone, the makeshift weapons, containers packed with explosives, fuel and scrap metal, have killed more than 250 people in Aleppo, including 73 children, according to the Britain-based Syrian Observatory for Human Rights.
Videos uploaded by activists showed the aftermath, including men weeping amid ravaged buildings and corpses covered with blankets on the pavement.
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"Be careful. There's a corpse under your feet. .. It's a child!" someone shouted. The videos were consistent with reporting by The Associated Press.
The rebels in Aleppo declared a push to seize the city's central prison and the Kweiras military air base to the east. Opposition fighters have been trying to capture the installations for months.
The Observatory and other activists said the rebels freed several hundred prisoners.
State-run Syrian television said the army foiled an attempt by "terrorist groups" to attack the prison.
Rebels have been besieging the prison, estimated to have 4,000 inmates, for almost a year. They have rammed suicide car bombs into the front gates twice, lobbed shells into the compound and battled frequently with the hundreds of guards and troops holed up inside.