By Natalia Zinets and Francesco Guarascio KYIV/BRUSSELS (Reuters) -Russian forces battled on Wednesday for control of the last stronghold in Ukraine’s besieged city of Mariupol, Ukrainian officials said, and the European Union proposed tough new sanctions against Moscow including a phased oil embargo. Mariupol authorities say some 200 civilians, as well as Ukrainian fighters, are still holed up in a vast network of underground bunkers in the Azovstal steel plant, which has suffered repeated bombardment from encircling Russian troops. Civilians who were evacuated to safety this week under a Uni…