By Pavel Polityuk KYIV (Reuters) -Ukrainian fighters were clinging to their last redoubt in Mariupol on Friday after Russian President Vladimir Putin claimed victory in the biggest battle of the war, declaring the port city “liberated” following weeks of relentless bombardment. The United States, however, disputed Putin’s claim and said it believed Ukrainian forces still held ground in the city. Putin ordered his troops to blockade a giant steel works where the Ukrainians are holding out, having refused an ultimatum to surrender or die. Ukraine said Putin wanted to avoid a final clash with its…