The bombings in Aleppo are part of a campaign by President Bashar Assad's forces to wrest control of neighborhoods that were seized by rebels in the northern city since mid-2012.
They also come against reports of talks between Syrian government and UN officials on extending the humanitarian truce in the central city of Homs to evacuate more people from the besieged rebel-held districts.
Khaled Erksoussi, the head of operations with the Syrian Arab red Crescent told The Associated Press that the agency's teams are on stand-by to accompany more people out of the city if the truce is extended. It is unlikely that aid workers will be able to deliver more aid to those still trapped in Old Homs, he said.
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The Britain-based Observatory for Human Rights said at least 51 people were killed yesterday, mainly by barrel bombs, shrapnel-packed explosive devices dropped on eight rebel-held districts from helicopters. The Observatory said that raised the total killed in the city to some 400 since the beginning of this month.
There is no way to independently verify the figure.
The Observatory, which has been documenting Syria's conflict since its start in March 2011 through a network of activists on the ground, released its report on the latest Aleppo casualties today, ahead of a trilateral meeting between senior US and Russian officials and UN mediator Lakhdar Brahimi, and Syrian government and opposition representatives in Geneva.
From the outset, the talks have been accompanied by a sharp rise in violence on the ground.