Russia is beefing up its forces for a new assault on Ukraine's eastern Donbas region, setting the stage for a protracted battle that is certain to inflict heavy losses on both sides as the Russians try to encircle Ukraine's fighters, analysts said.

Military analysts are wary of predicting who will win the battle for Ukraine's industrial heartland of Donbas, a crucial conflict that will likely be brutal and ultimately define the course of the war.