KYIV (Reuters) -Senior Ukrainian officials visited Washington on Tuesday seeking U.S. support against Russia, as Kyiv showed its ability to fight on by setting off an explosive device under a bridge that has become a symbol of the Kremlin's claims on Ukrainian territory. A day after talks in Istanbul that made little progress towards ending Russia's war in Ukraine, Kyiv launched what appeared to be one of its biggest waves of coordinated attacks of the conflict. Ukraine's SBU security service said it had hit a road and rail bridge that links Russia and Crimea below the water level with explosives.