By Pavel Polityuk and Elizabeth Piper KYIV (Reuters) -Ukraine said on Friday it was trying to break Russia’s siege of Mariupol as fighting raged around the city’s massive steel works and port, and the capital Kyiv was rocked by some of the most powerful explosions in two weeks. Ukrainian President Volodymyr Zelenskiy said the military situation in the south and east of the country was “still very difficult,” while praising the work of his armed forces. “The successes of our military on the battlefield are really significant, historically significant. But they are still not enough to clean our …