"Each and every barrel bomb filled with metal shrapnel and fuel launched against innocent Syrians underscores the barbarity of a regime that has turned its country into a super magnet for terror," Kerry was quoted as saying by Xinhua in a statement issued by the State Department.
"Each and every day that the barrel bombing of Aleppo continues, the Assad regime reminds the world of its true colours."
Syrian army dropped barrel bombs on a mosque in Aleppo on Tuesday, killing at least 11 people. Dozens of people have been killed by barrel bombs in the northwestern Syrian city over the past four days.
Accusing Assad's government of committing torture, using chemical weapons and blocking aid delivery, the top American envoy said that the Syrian people would "never accept as legitimate a government including Assad".
While the Syrian opposition and the international community are focused on ending the war, Assad's government is undermining hopes for the success of the peace process, Kerry added.
The first round of peace conference known as Geneva II ended in late January in Switzerland, without any meaningful solution to the conflict in Syria, which has killed more than 130,000 people in the past three years.