By Pavel Polityuk and James Mackenzie KYIV/KHARKIV (Reuters) – Russian troops seized a frontline Ukrainian city on Tuesday, hours after starting a long-anticipated offensive in eastern Ukraine that the Kyiv government has called the Battle of the Donbas. Thousands of troops pressed an advance across almost the entire stretch of the eastern front as the assault began with massive Russian artillery and rocket barrages. Ukrainian officials said their soldiers would withstand the offensive. In the ruins of Mariupol, the southeastern port that has withstood nearly eight weeks of siege, Russia gave …