John Kerry accused the Syrian government of carrying out "nothing short of a massacre" of their own citizens as he warned that east Aleppo must not become a bloodbath on the level of the genocide in Srebrenica.
The US Secretary of State said President Bashar Assad and his allies are preventing fresh peace negotiations that could bring an end to a brutal four-year conflict that has claimed hundreds of thousands of lives.
He also condemned the "continued, relentless and inexcusable attacks" on besieged residents and humanitarian workers inside besieged east Aleppo as they prepared to evacuate.
"There is absolutely no justification whatsoever for the indiscriminate and savage brutality against civilians shown by the regime and by it's Russian and Iranian allies," he said.
Mr Kerry said that President Assad and his allies were unleashing "a sectarian passion" and had carried out "indiscriminate slaughter" in an attempt to terrorise civilians into submission.
But the end of the battle for Aleppo provided a fresh opportunity to reflect and return to negotiations to bring an end to the conflict, he told a press conference.
"The killing and the suffering in Syria could stop very, very quickly if Russia and the regime made the decision to do so," he said.
He said that an earlier attempt at a ceasefire and roadmap to peace had mostly failed "because of the continued, constant unwillingness of the Assad regime to live by those agreements".
All parties in the war apart from the Syrian government and its allies had indicated they were willing to join fresh talks, he said.
The only remaining question is whether the Syrian regime was willing to go to Geneva and and "whether or not they are willing to stop this slaughter of their own people," he said.