KYIV/MARIUPOL (Reuters) -Ukraine said on Monday it had held off the latest assault on an eastern city that has become the main target of Moscow's offensive since Russian forces finally seized Mariupol last week. Russian forces tried to storm Sievierodonetsk, but were unsuccessful and retreated, President Volodymyr Zelenskiy's office said. The city, on the banks of the Siverskiy Donets river that winds through eastern Ukraine, has been the main Russian target in recent days as Moscow tries to encircle Ukrainian forces in the east and fully capture the Luhansk and Donetsk provinces. In Mariupol, where hundreds of Ukrainian fighters...