where russian is advancing most. they seized their first major city — kherson. and they've encircled mariupol, where residents are trapped from shelling. let's focus on south ukraine. russia is targeting these cities for strategic reasons. controlling the south cuts ukraine off from the sea — and creates a direct link between russia and crimea, which it anexxed in 2014. you can see from this map, troops are advancing from crimea. let's show you what mariupol looks like — it's had hours of sustained shelling. hundreds are feared dead. it's still under ukrainian control, but russian troops surround it. electricity, water and heat supplies have been cut. authorities say, "they are hindering food supplies, blocking us like in leningrad."