(Image source: ITV )   BY ZACH TOOMBS   Syrians in Aleppo Tuesday uncovered one of the most gruesome scenes yet in their nation’s two-year civil war.    Amateur video shows at least 70 bodies of young men and boys found along a riverbank — all of them blindfolded or bound and shot through the head in what appears to be a mass execution in the northwest of Syria. [Video: ITV ]   Al Jazeera reports from Aleppo, where relatives identifying the dead say there’s no doubt as to who’s responsible.   “The government says the victims were kidnapped and executed by what it calls terrorist groups, an accusation rejected by everyone here, who are calling this a new massacre.”   Since early 2011, rebels have taken up arms against Syrian President Bashar al Assad — who had cracked down violently on protesters and is now turning the full strength of his nation’s military against his own people — rebels his government labels terrorists. [Video: Sky News ]   The bodies were found Tuesday in the southwest Aleppo neighborhood of Bustan al-Qasr, which has seen especially heavy violence in the past few days. This amateur video posted Monday shows a skirmish between rebels and a government sniper.    No media outlet or international observer has determined who carried out the mass killing discovered Tuesday. And, as Al Jazeera reported, both rebels and the Assad regime blame one another. The BBC reports, as the civil war moves into its third year, battlelines are becoming more distinct.   “...the regime has been hitting back hard in many areas — around Damascus … It may have lost a lot of the outlying areas, but it’s clearly determined to hang onto the big cities and strike out from there.”