By Pavel Polityuk and James Mackenzie KYIV/KHARKIV (Reuters) – Russia was intensifying its attack on Ukraine’s east on Tuesday as it seized a frontline city and sought a decisive victory in Mariupol, prompting Western governments to pledge more arms and sanctions. Thousands of Russian troops backed by artillery and rocket barrages were advancing in what Ukrainian officials called the Battle of the Donbas. Russia’s nearly eight-week-long invasion has taken longer than many expected while still failing to capture any of the biggest cities, forcing Moscow to refocus in and around separatist regio…