<span style="color: #222222;">Russia gave Ukrainian fighters in Mariupol a fresh ultimatum to surrender today. Thousands of Russian troops backed by artillery and rocket barrages were advancing in what Ukrainian officials have called the Battle of the Donbas.<br />'' <br />'' Russia was hitting the Azovstal steel plant, the main remaining stronghold in Mariupol, with bunker-buster bombs, a Ukrainian presidential adviser said late last night.<br />'' <br />'' After an earlier ultimatum to surrender lapsed and as midnight approached, Russia's defence ministry said not a single Ukrainian soldier had laid down their weapons and it renewed the proposal. Ukrainian commanders have vowed not to surrender.<br />'' <br />'' The United States, Canada and Britain said they would send more artillery weaponry, and the White House said new sanctions were being prepared.<br />'' <br />''<br />''U.N. Secretary-General Antonio Guterres called for a four-day humanitarian pause in the fighting this coming weekend, when Orthodox Christians celebrate Easter, to allow civilians to escape and humanitarian aid to be delivered.</span>