Backers of the Syrian opposition appear resigned to the fall of eastern Aleppo, with the United States pleading Moscow to show "grace" when officials meet in Geneva to try to reach a deal for civilians and fighters to leave the city.
Syrian government and allied forces have in the last two weeks driven rebels from most of their territory in what was once Syria's most populous city.
The rebels have controlled the eastern section since 2012, and President Bashar al-Assad said in an interview published on Thursday that retaking Aleppo would change the course of the civil war across the whole country.
Critics of the operation have warned that thousands of civilians risk being caught in the crossfire and have repeatedly called on Syrian government forces and its backers to accept a ceasefire to at least allow corridors for civilians and rebel fighters to leave to safer areas.
Speaking in Paris after a meeting of countries that oppose Assad, including France, Britain, Turkey and Saudi Arabia, an exasperated Kerry showed little optimism for US and Russian talks in Geneva later on Saturday.
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"Our teams are meeting in Geneva today in order to flesh out details of a possible way of trying to save lives. Russia and Assad have a moment where they are in a dominant position to show a little grace," Kerry told reporters.
"I believe there could be a way forward but it depends on big, magnanimous choices from Russia ... and insistence of Russia on the Assad regime," Kerry said.
Moscow and Washington have discussed a ceasefire to let civilians escape eastern Aleppo and aid enter.
Russia also wants the United States to urge rebel fighters to abandon their territory and accept transport out.
"Fighters ... don't trust that if they agreed to leave to try to save Aleppo that it will save Aleppo and they will be unharmed and free to move where they are not immediately attacked," he said.
Rebel fighters now believed their choice was to either "die in Aleppo or die in Idlib, but die," Kerry said referring to the rebel-held city in northwestern Syria.
