A suicide bomber blew himself up at the gates of a Syrian prison and rebels stormed in behind him, freeing hundreds of inmates as part of an offensive aimed at capturing key government symbols in and around the northern city of Aleppo, activists said.
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.
They include at least 11 who died Thursday - six of them from the same family - in the opposition-held neighbourhood of Masaken Hanano.
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 news agency.
UN Secretary-General Ban Ki-moon deplored "the ongoing aerial attacks and the use of "barrel bombs" to brutal, devastating effect in populated areas" which violate international humanitarian and human rights law, UN spokesman Martin Nesirky said.
In other developments, Syrian President Bashar al-Assad's government said it has reached an agreement with the United Nations to let hundreds of trapped civilians leave besieged parts of the city of Homs and permit UN humanitarian relief convoys to enter.
In New York, UN deputy spokesman Farhan Haq said the United Nations welcomed reports that the parties agreed to "a humanitarian pause".
He said the UN and humanitarian organisations have food, medical aid and other basic supplies on the outskirts of Homs ready for immediate delivery as soon as "the green light" is given for safe passage.
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.
Thursday's offensive began when a Chechen suicide bomber from the al-Qaida-linked Nusra Front attacked the prison gates, according to the Observatory.
Rebel fighters then managed to gain control of large parts of the compound. By evening, heavy clashes between the rebels and soldiers were raging inside.
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.
The nearly 3-year-old uprising against Assad has left more than 130,000 people dead and forced more than 2.3 million to seek refuge abroad.