The United States is trying to determine whether a Russian plan for a humanitarian operation in Syria is sincere, Secretary of State John Kerry said on Friday, adding that if it proves a "ruse" it could ruin cooperation between Moscow and Washington.
The 250,000 civilians trapped for weeks inside the besieged rebel-held sector of Aleppo have so far stayed away from "safe corridors" that Moscow and Damascus promised for those trying to escape the most important opposition stronghold in the country.
Syrian President Bashar al-Assad's government and its Russian allies declared a joint humanitarian operation for the besieged area on Thursday, bombarding it with leaflets telling fighters to surrender and civilians to leave.
The United Nations has raised misgivings about the plan and U.S. officials have suggested it may be an attempt to depopulate the city so that the army can seize it. The Syrian opposition called it a euphemism for forced displacement of the inhabitants, which it said would be a war crime.
Aleppo, Syria's biggest city before the war, has been divided since 2012 into government and rebel sectors. Seizing control would be the biggest victory for Assad in five years of fighting, and demonstrate the dramatic shift of fortunes in his favor since Moscow joined the war on his side last year.