Ukraine reinforced its defenses at the port of Mariupol on the Sea of Azov on Thursday in anticipation of a rebel attack amid reports that pro-Russian separatists were advancing on the city with tanks and artillery. Further north, in the rebel stronghold of Donetsk, renewed shelling killed at least one woman and blew huge holes in residential buildings, a day before talks between Kiev and the separatists on a possible ceasefire. A Ukrainian soldier told Reuters he had seen the separatists advancing on Mariupol, a city of about 500,000, with tanks, armored personnel carriers and artillery. His comments could not be independently confirmed. A Reuters witness heard about a dozen blasts and saw plumes of black smoke rising a few km (miles) to the east of Mariupol.