By Steve Holland and Pavel Polityuk WASHINGTON/LVIV, Ukraine (Reuters) – U.S. President Joe Biden on Tuesday announced a ban on Russian oil and other energy imports, a significant move in piling pressure on President Vladimir Putin to halt his devastating assault on Ukraine. Ukraine’s government accused Russian forces of shelling a humanitarian corridor that Moscow had promised to open to let residents flee the besieged port of Mariupol. The civilian death toll in the conflict mounted. And with the war in its 13th day, the number of refugees who have fled Ukraine to neighbouring countries surg…