Russia is grinding its way into Kostiantynivka, a key stronghold in Ukraine’s eastern “fortress belt” long coveted by the Kremlin, even as its gains across the rest of the 1,200-kilometre front line have largely stalled. Fighting has begun to seep into the city itself. Small groups of Russian soldiers are attempting to infiltrate its outskirts, senior Ukrainian commanders said last week, suggesting close-quarters assaults could follow.