(Adds unconfirmed reports of chemical weapons use in Mariupol) By Maria Starkova LVIV, Ukraine (Reuters) – Ukraine said on Monday it expects Russia to begin an offensive soon in the eastern Donbas region as Moscow shifts its focus to seizing territory there after its invasion force was driven from the gates of Kyiv this month. Russia’s main target in Donbas is the port of Mariupol, where thousands are believed to have died under a near-seven week siege. Russia is amassing thousands of troops in the area for a new assault, Ukrainian President Volodymyr Zelenskiy said. The first European Union l…