Kerry warns Aleppo must not become new Srebrenica

US Secretary of State John Kerry warned Thursday that tens of thousands of Syrian civilians are still trapped in east Aleppo and must not face a Srebrenica-style massacre.

Washington's top diplomat expressed outrage at the fate of the city, but offered no new plan to end the civil war, demanding that Syrian President Bashar al-Assad agree to peace talks.

"What has happened already in Aleppo is unconscionable," Kerry told reporters as a first convoy of hundreds Aleppo civilians made use of a ceasefire to flee the city.

"But there remains tens of thousands of lives that are now concentrated into a very small area of Aleppo," he said.

"And the last thing anybody wants to see... is that that small area turns into another Srebrenica," he said, referring to a 1995 Bosnian war massacre.

Kerry said he had been in recent contact with all the major international players in the conflict about reviving the idea of Geneva talks between Assad and rebel leaders.

And he demanded that Russia, which backs Assad and which he again accused of complicity in attacks on civilians, compel its ally to come to the negotiating table.

"The only remaining question is whether the Syrian regime with Russia's support is willing to go to Geneva prepared to negotiate constructively," he said.

He accused Assad, who is from Syria's Alawite minority and is backed by Iranian-backed Shia militias, of unleashing a "sectarian passion" in his Sunni majority country.

"The Assad regime is actually carrying out nothing short of a massacre," Kerry said.

"And we have witnessed indiscriminate slaughter, not accidents of war, not collateral damage, but frankly purposeful, a cynical policy of terrorizing civilians."

He called on the Assad regime and the Russian military to make a "strategic decision " for peace.