Leningrad was able to hold out so long because 1) they resorted to eating corpses instead of burying them, and 2) they were not 100% cut off, in the winter season they were resupplied by trucks driving over a frozen lake. Not familiar with Sarajevo so I can't comment on that. But unless Kiev is willing to resort to extreme measures, they likely will fall once surrounded.
They could possibly keep Kiev resupplied from the air, similar to the Berlin Airlift that the Allies did to keep commies from starving West Berlin. But unlike the Berlin airlift they would be doing it under constant enemy fire from the ground and air.
Keeping Kiev is not impossible for Ukraine, but imo once Kiev is surrounded, they're only keeping it if the Russians let them.