Ukrainian commander in Mariupol pleads for help: "We are probably facing our last days, if not hours"

Ukraine said a preliminary agreement was reached to establish a humanitarian corridor and evacuate thousands of people from the besieged coastal city of Mariupol on Wednesday, as Russia hurled its military might against Ukrainian cities and towns in an attempt to slice the country in two and gain control of the eastern industrial heartland of coal mines and factories.

The fighting unfolded along a boomerang-shaped front of hundreds of miles in what is known as the eastern Donbas region. If successful, Russian President Vladimir Putin would be able to claim a victory following his failed attempt to storm the capital, Kyiv, and heavier-than-expected Russian casualties.

"We are probably facing our last days, if not hours"

Key to the Russian campaign is the capture of the city of Mariupol, which would deprive Ukraine of a vital port and complete a land bridge between Russia and the Crimean Peninsula, which Russia seized from Ukraine in 2014. It would also free up Russian troops to move elsewhere in the Donbas region.

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.

Fighting rages on

The eastern cities of Kharkiv and Kramatorsk also came under deadly attack. Russia said it struck areas around Dnipro and Zaporizhia with missiles.

Near Zaporizhia, 29-year-old paratrooper Vyacheslav Dimovon was laid to rest after a Russian anti-tank mine took his life on Saturday. His mother, Alla, told CBS News' Charlie D'Agata how important it had been for Dimovon to fight for his country.

"He dreamed like crazy of becoming a soldier," Alla said. "But I didn't bring him into this world for war, but for a peaceful life. So he could live with his beloved wife Ivanna."

Dimovon and his 23-year old widow had only been married for 18 months. They were planning to start a family when the war broke out.

"Every time we spoke, I told him: 'I love you, look after yourself,'" Ivanna told CBS News. She said she had known every video call with her husband could be their last.

"'Now, it's: 'I love you, look after me,'" she said.

Both sides have described the Russian assault that began Monday as a new phase of the war.

Ukrainian President Volodymyr Zelenskyy said the Russian military was throwing everything it has into the battle, with most of its combat-ready forces now concentrated in Ukraine and just across the border in Russia.

"They have driven almost everyone and everything that is capable of fighting us against Ukraine," he said in his nightly video address to the nation.

Despite claims that they are hitting only military sites, the Russians continue to target residential areas and kill civilians, he said. He also said the Kremlin has not responded to a proposal to exchange Viktor Medvedchuk, the jailed leader of a pro-Russia party, for the Mariupol defenders.

A senior U.S. defense official, speaking on condition of anonymity to discuss the Pentagon's assessments of the war, said the Russians had added two combat units, known as battalion tactical groups, in Ukraine over the preceding 24 hours. That brought the total units in the country to 78, all of them in the south and the east, up from 65 last week, the official said.

That would translate to 55,000 to 62,000 troops, based on what the Pentagon said at the start of the war was the typical unit strength of 700 to 800 soldiers. But accurately determining Russia's fighting capacity at this stage is difficult.

A European official, likewise speaking on condition of anonymity to discuss military assessments, said Russia also has 10,000 to 20,000 foreign fighters in the Donbas region. They are a mix of mercenaries from Russia's private Wagner Group and Russian proxy fighters from Syria and Libya, according to the official.