US Secretary of State John Kerry urged Russia to show "a little grace" when US and Russian officials meet in Geneva later to try to reach a deal enabling civilians and fighters to leave the besieged city of Aleppo.
"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," Mr Kerry told reporters in Paris after a meeting of countries opposed to Syrian President Bashar al-Assad.
"Russia and Assad have a moment where they are in a dominant position to show a little grace," he said, adding that the talks in Geneva were aimed at finding a possible way of trying to save lives.
He also said that Syrian regime's "indiscriminate bombing" of Aleppo amounted to crimes against humanity and war crimes and called on Russia to try to stop it.
"The indiscriminate bombing by the regime violates rules of law, or in many cases, crimes against humanity, and war crimes," he Kerry said and called for Russia to "do their utmost to bring it to a close".
Turkish Foreign Minister Mevlut Cavusoglu said that Syrian rebel forces risk being killed elsewhere if they left Aleppo, and doubted that the talks in Paris would yield concrete results.
Turkish-backed forces are now besieging al-Bab, the last urban stronghold of the so-called Islamic State in the northern Aleppo countryside.
Their advance potentially pits the Turks against both Kurdish fighters and Syrian government forces.
More than 20,000 civilians have left eastern Aleppo and over 1,200 rebels laid down their weapons, Interfax news agency cited the Russian Defence Ministry as saying this morning.
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US to send 200 military personnel to Syria
The United States is sending 200 additional military personnel to Syria to help the campaign to drive the so-called Islamic State from Raqqa.
US Defense Secretary Ash Carter said the 200, including special forces trainers, advisors and explosive ordnance disposal teams, would join 300 US special force troops already in Syria.
Speaking in Bahrain at the Manama Dialogue conference on Middle East security, Mr Carter said that Russia, President Bashar al-Assad's top foreign military backer, had "only inflamed the civil war and prolonged suffering of the Syrian people."
He added that the first goal of a coalition opposed to the militants was to "destroy the ISIL cancer's parent tumour in Iraq and Syria, because the sooner we crush both the fact and the idea of an Islamic state based on ISIL's barbaric ideology, the safer we'll all be", he said, referring to Islamic State.
Syria's civil war pits President Assad, backed by Iran, Russia and some Shia militias, against mostly Sunni Arab rebels backed by Turkey, Gulf monarchies and the United States.
A secondary conflict puts all of them at war with Islamic State, an effort that coincides with a drive against the group in Iraq.
The Iraqi city of Mosul and the smaller Syrian city of Raqqa are the two pillars of Islamic State's self-declared caliphate, and recapturing them would be a pivotal defeat for the ultra-hardline Sunni jihadists.
