"Each and every day that the barrel-bombing of Aleppo continues, the Assad regime reminds the world of its true colors," Kerry wrote in a statement yesterday.
"It is the latest barbaric act of a regime that has committed organized, wholesale torture, used chemical weapons, and is starving whole communities by blocking delivery of food to Syrian civilians in urgent need."
More than 150 people have been killed in Syria's onetime economic hub and second city over the past four days, in a string of barrel bomb raids and other air strikes, the Syrian Observatory for Human Rights said.
The bombs hit a mosque, which the Aleppo media center said was being used as a school.
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"Given this horrific legacy, the Syrian people would never accept as legitimate a government including Assad," Kerry said, referring to peace talks due to resume in Geneva next week aimed at installing a transitional government in the war-torn country.
"While the opposition and the international community are focused on ending the war... The regime is single-mindedly focused on inflicting further destruction to strengthen its hand on the battlefield and undermining hopes for the success of the Geneva II process," Kerry said.