Russian troops are pressing their offensive across Ukraine, pounding populated areas with artillery and airstrikes and deploying siege tactics honed in Syria and Chechnya — where front line cities were reduced to rubble. The Ukrainian government said Saturday that Russia’s troops have shelled a mosque sheltering more than 80 people in the besieged city of Mariupol. There was no immediate word on casualties. Bombardment intensified as Russian units fanned out to prepare their assault on Ukraine’s capital, Kyiv, a major political and strategic prize for Russian President Vladimir Putin. Ukrainia…