Speaking as officials from the US, Russia and Syria's neighbours met in Geneva to seek ways to restore a ceasefire in the civil war, Kerry said yesterday that Russia's bombardment of rebel-held areas was a mistake.
Moscow argues that it is supporting Syrian leader Bashar al-Assad against Al-Qaeda-linked "terrorists" and blames Washington for failing to separate moderate rebels from the hardliners.
But, speaking in Washington after talks with his South Korean counterpart, Kerry said: "Every bomb that's dropped by Russia and the Assad regime is radicalising more and more people."
"If Russia and Assad succeed in taking Aleppo, the fundamental dynamic of this war does not change," Kerry said, warning Russia to recommit to the search for a viable ceasefire deal.
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"If you don't have a political settlement you can't have peace and so Russia needs to understand this is not making things better, it's making things worse.
"And we have put on the table an opportunity for Russia to make a different choice, a simple choice," he continued.