By Natalia Zinets and Tom Balmforth KYIV (Reuters) -Russian troops bombarded a riverside city on Friday in what appeared to herald a major assault to seize the last remaining Ukrainian-held territory in a province Moscow claims on behalf of separatists. Ukrainian officials said Russian forces had launched a massive artillery bombardment against Sievierodonetsk, one of the last Ukrainian-held bastions in Luhansk, one of two southeastern provinces Moscow and its proxies proclaim as independent states. The city, and its twin Lyshchansk on the opposite bank of the Siverskiy Donets river, form the …