KYIV, UKRAINE, APRIL 20

Russian forces pressured a stubborn pocket of resistance in Mariupol amid renewed hopes Wednesday for an evacuation of thousands of civilians from the shattered port city that is a key battleground in Moscow's new onslaught to take control of Ukraine's eastern industrial heartland.

In addition to pounding the holdout in Mariupol, Russian forces have intensified their attacks along a boomerang-shaped front hundreds of miles long elsewhere in the area known as the Donbas, home to coal mines, metal plants and factories vital to Ukraine's economy.

If successful, the offensive would carve Ukraine in two and give President Vladimir Putin a badly needed victory following the failed attempt by Moscow's forces to storm the capital, Kyiv, and stronger-than-expected resistance in the nearly two-month war.

But analysts say it could also devolve into a grim war of attrition as Russia attempts to defeat Ukraine's most experienced, battle-hardened troops who already have been fighting pro-Moscow separatist forces for eight years in the mostly Russian-speaking Donbas.

With that potentially pivotal offensive underway, Russia said Wednesday 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 told a conference call with reporters that "the ball is in (the Ukrainians') court, we're waiting for a response." He gave no details on the draft, and it was not clear when the Russian document was sent or if it offered anything new to the Ukrainians, who presented their own demands last month.

A Ukrainian presidential adviser said that Kyiv was reviewing the proposals.

Ukraine's ability to bog Russia down has been on display at Mariupol, where a siege since the early days of the war has flattened much of the city but not yet resulted in a full victory.

The port on the Sea of Azov has been the scene of some of the most dramatic suffering of the conflict, which has left thousands dead, pushed more than 5 million people to flee the country, according to the U.N., displaced millions more inside it, and upended the post-Cold War security balance in Europe.

Ukrainian troops said Tuesday the Russian military dropped heavy bombs to flatten what was left of a sprawling steel plant - believed to be the last holdout of troops defending Mariupol - and hit a makeshift hospital where hundreds were staying. The reports could not be independently confirmed.

Serhiy Taruta, the former governor of the Donetsk region and a Mariupol native, said 300 people, including wounded troops and civilians with children, were sheltered at the hospital.

Deputy Prime Minister Iryna Vereshchuk, meanwhile, said that there was a "preliminary" agreement to open a humanitarian corridor for women, children and the elderly to leave Mariupol and head west to the Ukraine-controlled city of Zaporizhzhia on Wednesday afternoon.

Mariupol Mayor Vadym Boychenko urged locals to leave the city, though previous such agreements have fallen apart, with Russians preventing buses meant to pick up evacuees from entering the city or shelling escape routes.

"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," he wrote in a statement issued by the city council.

Officials have given varying estimates of how many people remain in the city, which had a pre-war population of more than 400,000.

Boychenko said about half had fled and asked those who had to contact relatives still in the city and urge them to evacuate, saying buses would be provided and one pickup point would be near the Azovstal steel mill. A Ukrainian police official has said civilians, including children, are sheltering there among the city's last known defenders.

Many previous evacuation efforts relied on civilians using private cars after efforts to bring buses into the city failed. But with fuel supplies and the number of cars dwindling in the city, that is becoming increasingly difficult.

There was no immediate confirmation on the evacuation from the Russian side, which issued a new ultimatum to the Ukrainian defenders to surrender Wednesday. The Ukrainians have ignored previous demands to leave the sprawling steel plant's warren of tunnels and bunkers.

Capturing Mariupol holds strategic and symbolic value for both sides. The scale of suffering there has made it a focal point of the war for many outside Ukraine, and Russia's difficulty in definitively taking it is a prime example of the ways an under-gunned Ukrainian force has stymied Moscow's troops.

Mariupol's fall would also deprive Ukraine of a vital port, complete a land bridge between Russia and the Crimean Peninsula that Moscow seized from Ukraine in 2014 and free up Russian troops to move elsewhere in the Donbas.

A few thousand Ukrainian troops, by the Russians' estimate, remained holed up in the steel plant.

The General Staff of Ukraine's military said in a statement that taking control of the Azovstal mill and thus fully capturing Mariupol remains a top Russian priority. But it added that Moscow's forces were continuing to mount offensives across the east as its forces probe for weak points in the Ukrainian defensive lines.

The Russian Defense Ministry said its forces launched hundreds of missile and air attacks on Ukrainian targets. Ministry spokesman Igor Konashenkov also said there had been missile strikes on concentrations of Ukrainian troops and vehicles in the Kherson Region in southern Ukraine. Those claims could not be independently verified.

Both Russian and Ukrainian officials have described stepped-up assaults along a broad front in the east that began Monday as a new phase of the war.

Putin used a public appearance on Wednesday to push his false narrative of the war, saying it was necessary to protect people living in the Donbas - referring to them as "our people."

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.

Military experts said the Russians' goal is to encircle Ukrainian troops in the Donbas from the north, south and east.

Eyewitness accounts and reports from officials have given a broad picture of the extent of the Russian advance. But independent reporting in the parts of the Donbas held by Russian forces and separatists is severely limited, making it difficult to know what is happening in many places on the ground.

Western nations, meanwhile, are boosting their donations of military supplies to Kyiv.

US President Joe Biden is expected to announce a new weapons package in the coming days that will include additional artillery and ammunition, according to a US official, who was not authorized to comment publicly and spoke on the condition of anonymity.