Russia's humanitarian plan for up to 300,000 civilians besieged in the Syrian city of Aleppo should be improved, U.N. Syria envoy Staffan de Mistura said on Friday, suggesting Moscow should consider handing over responsibility for any evacuation to the United Nations."My understanding is the Russians are open for major improvements," Staffan de Mistura told reporters. He said U.N. humanitarian experts knew what to do and had the necessary experience."That's our job," he said.