By Pavel Polityuk KYIV/MARIUPOL, Ukraine (Reuters) – Russian air raids and missile strikes hit Ukraine’s capital Kyiv and other major cities on Saturday as Moscow launched more long-range attacks following the sinking of its Black Sea fleet’s flagship. In the besieged port of Mariupol, scene of the war’s heaviest fighting and worst humanitarian catastrophe, Russian troops pressed their advances, hoping to make up for their failure to capture Kyiv by seizing their first big prize of the war. “The situation is very difficult” in Mariupol, President Volodymyr Zelenskiy told the Ukrainska Pravda n…