KYIV, Ukraine -- Russian forces tightened the noose around the defenders holed up Wednesday in a mammoth steel plant that represented the last known Ukrainian stronghold in Mariupol, as a fighter apparently on the inside warned in a video plea for help: "We may have only a few days or hours left."
With the holdouts coming under punishing new bombing attacks, Ukrainian authorities struggled to open an evacuation corridor for people trapped in the ruined port city, a key battleground in Moscow's drive to seize the country's industrial east.
Meanwhile, the Kremlin said it submitted a draft of its demands for ending the fighting, the number of people fleeing the country climbed past 5 million, and the West raced to supply Ukraine with heavier weapons for the potentially grinding new phase of the war.
With global tensions running high, Russia reported the first successful test launch of a new type of intercontinental ballistic missile, the Sarmat. President Vladimir Putin boasted it can overcome any missile defense system and make those who threaten Russia "think twice," and the head of the Russian state aerospace agency called the launch out of northern Russia "a present to NATO."
The Pentagon described the test as "routine" and said it wasn't considered a threat.
On the battlefield, Ukraine's military said Moscow continued to mount assaults across the east, probing for weak points in Ukrainian defensive lines. Russia said it launched hundreds of missile and air attacks on targets that included concentrations of troops and vehicles.
The Kremlin's stated goal is the capture of the Donbas, the mostly Russian-speaking eastern region that is home to coal mines, metal plants and heavy-equipment factories vital to Ukraine's economy. Detaching it would give Putin a badly needed victory two months into the war, after the botched attempt to storm the capital, Kyiv.
Analysts say the offensive in the east could devolve into a grim war of attrition as Russia runs up against Ukraine's most experienced, battle-hardened troops, who have been fighting pro-Moscow separatists in the Donbas for the past eight years.
With the potentially pivotal offensive underway, Russia said it has presented Ukraine with a draft document outlining its demands as part of talks aimed at ending the conflict - days after Putin said the negotiations were at a "dead end."
Kremlin spokesman Dmitry Peskov said that "the ball is in their court, we're waiting for a response." He gave no details on the draft, and it was not clear when it was sent or if it offered anything new to the Ukrainians, who presented their own demands last month.
A Ukrainian presidential adviser said Kyiv was reviewing the proposals.
Moscow has long demanded, among other things, that Ukraine drop any bid to join NATO. Ukraine has said it would agree to that in return for security guarantees from a number of other countries.
In devastated Mariupol, Ukraine said the Russians dropped heavy bombs to flatten what was left of the sprawling Azvostal steel plant, believed to be the last pocket of resistance in the city. A few thousand Ukrainian troops, by the Russians' estimate, remained in the plant and its labyrinth of tunnels and bunkers encompassing about 11 square kilometers (4 square miles).
On Wednesday, a Ukrainian posted a video plea on Facebook urging world leaders to help evacuate people from the plant, saying, "We have more than 500 wounded soldiers and hundreds of civilians with us, including women and children."
The officer, who identified himself as Serhiy Volynskyy of the 36th Marine Brigade, said: "This may be our last appeal. We may have only a few days or hours left." The authenticity of the video could not be independently verified.
The Russian side issued a new ultimatum to the defenders to surrender Wednesday, but the Ukrainians have ignored previous demands to leave the plant's labyrinth of tunnels and bunkers.
All told, more than 100,000 people were believed trapped in Mariupol, which had a pre-war population of over 400,000.
Deputy Prime Minister Iryna Vereshchuk said there was a preliminary agreement to open a humanitarian corridor for women, children and the elderly to leave and head west to the Ukraine-controlled city of Zaporizhzhia on Wednesday afternoon.
Mariupol Mayor Vadym Boychenko urged residents to leave, though previous such agreements have fallen apart, with the Russians shelling escape routes or otherwise preventing buses meant to pick up evacuees from entering the city.
"Do not be frightened and evacuate to Zaporizhzhia, where you can receive all the help you need - food, medicine, essentials - and the main thing is that you will be in safety," the mayor said in a statement.
Mariupol holds strategic and symbolic value for both sides. The scale of suffering there has made it a worldwide focal point of the war. Mariupol's fall would deprive Ukraine of a vital port, complete a land bridge between Russia and the Crimean Peninsula, which Moscow seized from Ukraine in 2014, and free up Russian troops to move elsewhere in the Donbas.
As Russia continued to funnel troops and equipment into the Donbas, Western nations rushed to boost the flow of military supplies to Kyiv for this new phase of the war, which is likely to involve trench warfare, long-range artillery attacks and tank battles across relatively open terrain.
U.S. President Joe Biden is expected to announce a new weapons package in the coming days that will include additional artillery, and Canada and the Netherlands also said they would send more heavy weapons.
Also, a senior U.S. defense official, speaking on condition of anonymity to discuss the Pentagon's assessment of the war, said the training of Ukrainian personnel on American 155 mm howitzers has begun in a European country outside Ukraine, and the first of 18 promised such weapons began arriving on the continent.
Putin, meanwhile, hailed the launch of the Sarmat as "a big, significant event" for Russia's defense industry and praised the missile as having "no equivalents in the world." The Sarmat is intended to eventually replace the Soviet-built missile code-named Satan by NATO as a major component of Russia's nuclear arsenal.
"This truly unique weapon will strengthen the combat potential of our armed forces, reliably ensure Russia's security from external threats and make those who, in the heat of frantic, aggressive rhetoric, try to threaten our country think twice," Putin said.
Pentagon spokesman John Kirby said Russia notified the U.S. that it planned to test the missile, and "we did not deem the test to be a threat to the United States or its allies."
Looking for a path to peace, U.N. Secretary-General Antonio Guterres requested meetings with both Putin and Ukrainian President Volodymyr Zelenskyy in their respective capitals to discuss "whatever urgent steps can be taken to stop the fighting," spokesperson Stephane Dujarric said. The U.N. received no immediate response.
'Days or hours left': Russia tightens the noose in Mariupol
The fighting unfolded along a boomerang-shaped front hundreds of miles long in what is known as the Donbas.