BEIRUT — At least 51 people, including 13 rebels, died in a single day of Syrian government airstrikes and shelling of opposition-controlled districts of Aleppo, activists said today as international mediators were trying to salvage faltering peace negotiations in Geneva.

The Britain-based Observatory for Human Rights said most of the victims were killed by airstrikes and barrel bombs dumped on eight rebel-held districts of the northern city on Wednesday. The rest died from artillery shelling, sniper fire and in clashes between forces loyal to President Bashar Assad and rebels fighting to overthrow him.

The bombings in Aleppo, Syria’s largest city, are part of a campaign by Assad’s forces to wrest control of neighborhoods that were seized by rebels in mid-2012.

The Observatory has been documenting Syria’s conflict since its start in March 2011.