Ukrainian troops said the Russian military dropped heavy bombs to flatten what was left of a sprawling steel plant in Mariupol - believed to be the city's defenders' last holdout, and hit a hospital where hundreds were staying.

    Deputy Prime Minister Iryna Vereshchuk said there was a "preliminary" agreement to open a humanitarian corridor for women, children and the elderly to leave Mariupol west to the Ukraine-controlled city of Zaporizhzhia on Wednesday afternoon. She said on the messaging app Telegram that civilians in Mariupol were facing a "catastrophic humanitarian situation."

    Vereshchuk previously said no agreement had been reached with Russia on an evacuation route on each of the past three days. There was no immediate confirmation from the Russian side, which issued a new ultimatum to the Ukrainian defenders to surrender Wednesday after a previous ultimatum was ignored. Ukraine and Russia have frequently blamed each other for obstructing evacuations from Mariupol or firing along the agreed route, which has typically only been open to people in private vehicles.

    "We are probably facing our last days, if not hours," a commander of the Ukrainian marines fighting in the steel plant, Serhiy Volyna, said in a video posted on social media. "The enemy is outnumbering us 10 to 1. They have advantage in the air, in artillery, in their forces on land, in equipment and in tanks. We are only defending one object, the Azovstal plant where, in addition to military personnel, there are also civilians who have fallen victim to this war. We appeal and plead to all world leaders to help us. We ask them to use the procedure of 'extraction' and take us to the territory of a third party state. All of us, Mariupol military battalion of soldiers, more than 500 wounded, and hundreds of civilians including women and children, we plead to take us to safety on the territory of a third party state," Volyna said.

   "I can confirm that there are a lot of civilians," Deputy Mayor of Mariupol Sergei Orlov told CBS News partner network BBC News. "They know that steel plants had good bomb shelter and some stocked food and water in the bomb shelter. That's why they decided with their families to live in this bomb shelter."

    Orlov said no help was currently possible for the people in the steel plant.

        "They have an absolute lack of everything. A lack of water, food, medicines, help, and Russia totally blocks everything, any humanitarian help or evacuation," Orlov said.