Russia issued Ukrainian fighters holding out in Mariupol a new ultimatum to surrender by Wednesday afternoon as it intensified its offensive in Ukraine’s eastern industrial heartland while Western countries pledged to provide more military aid to Ukraine in order to help resist the invasion.

Thousands of Russian troops backed by artillery and rocket barrages were attempting to advance in what Ukrainian officials have called the "Battle of the Donbass" – a final push by Moscow to seize two eastern provinces it claims on behalf of separatists.

Russia's nearly eight-week invasion has failed to capture any of Ukraine's largest cities. Moscow was forced to retreat from northern Ukraine after an assault on Kyiv was repelled last month, but has poured troops back in for an assault on the east that began this week.

The biggest attack on a European state since 1945 has led to nearly 5 million people fleeing abroad and reduced cities to rubble. In the ruins of Mariupol, the site of the war's heaviest fighting and worst humanitarian catastrophe, Russia was hitting the last main Ukrainian stronghold, the Azovstal steel plant, with bunker-buster bombs, Kyiv said. Ukraine says hundreds of civilians are sheltering beneath the factory.

"The world watches the murder of children online and remains silent," presidential adviser Mykhailo Podolyak wrote on Twitter.

Russia has been trying to take full control of Mariupol since the war's first days. Its capture would be a big strategic prize, linking territory held by pro-Russian separatists in the east with the Crimea region that Moscow annexed in 2014.

But not a single Ukrainian soldier had laid down their weapons after an ultimatum to surrender lapsed on Tuesday, Russia's Defense Ministry said. It announced a new deadline of 2 p.m. on Wednesday (11 a.m. GMT) for defenders to lay down arms. The new Russian ultimatum to Ukrainian troops in Mariupol expired Wednesday afternoon with no mass capitulation.

Ukraine has vowed never to surrender in Mariupol and its general staff said fighting was continuing at the plant.

A commander in the besieged Azovstal power plant, on the other hand, issued a desperate plea for help, saying his marines were "maybe facing our last days, if not hours."

"The enemy is outnumbering us 10 to one," Serhiy Volyna from the 36th Separate Marine Brigade said.

"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."

Thousands of troops and civilians remain holed up in the plant.

Ukraine announced plans to send 90 buses to evacuate 6,000 civilians from Mariupol on Wednesday, saying it had reached a "preliminary agreement" with Russia on a safe corridor, the highest profile announcement of such a plan for weeks. Moscow has blocked all previous convoys to Mariupol, including one sent by the Red Cross at the end of March.

Civilians have been able to escape to other parts of Ukraine only in their own vehicles, while tens of thousands have been bussed to Russia in what Moscow calls humanitarian evacuation and Kyiv calls illegal forced deportation.

Once a prosperous port of 400,000 people, Mariupol has been turned into a blasted wasteland with corpses in the streets, and residents confined to cellars. Ukrainian officials say tens of thousands of civilians have died there.

Battle of Donbass

The battle for the Donbass region could be decisive as Russia searches for a victory to justify President Vladimir Putin's Feb. 24 invasion.

British military intelligence said fighting in Donbass was intensifying as Russian forces tried to break through Ukrainian lines and disrupt its reinforcements and that Russia was still building up forces on Ukraine's eastern border.

Moscow is hoping its advantage in firepower will give it more success against Ukrainian defenders than in the failed campaign against Kyiv, when its overstretched supply lines were attacked by nimble small units. Kyiv has recently attacked Russian supply lines near the eastern city of Kharkiv.

Russian forces captured Kreminna, a frontline town of 18,000 people, on Tuesday and Ukraine's general staff said Russian forces had attempted an offensive near Kharkiv, the country's second-biggest city.

Inside Kharkiv, where at least four people were killed in missile strikes on Tuesday, the body of an elderly man lay face down near a park on a suburban street, a ribbon of blood running into the gutter.

"He worked in security not far from here," a resident named Maksym told Reuters. "The shelling began and everyone fled. Then we came out here, the old guy was already dead."

European Council head Charles Michel arrived in Kyiv as the latest European official to visit and demonstrate support.

Michel said Wednesday that there must be justice for war crimes committed in Ukraine as he toured the devastated town of Borodyanka on a visit to the country.

"In Borodianka. Like Bucha and too many other towns in Ukraine. History will not forget the war crimes that have been committed here. There can be no peace without justice," Michel wrote on Twitter.

Michel is the latest foreign leader to head to Kyiv in a show of support for the pro-Western authorities as they battle the onslaught from Moscow's forces.

Kyiv has asked its Western partners to provide MiG-29s, which its pilots already know how to fly and a handful of Eastern European countries have. The West has responded to a renewed Russian push into the Donbass region with fresh weapons for Kyiv and a push to increase "Moscow's international isolation."

The United States, Canada and Britain said they would send Ukraine more artillery, while Norway said it had shipped Ukraine 100 Mistral air defense missiles. The Pentagon said that Ukraine had recently received fighter planes and parts to bolster its air force, declining to specify the number of aircraft and their origin.

The White House said new sanctions against Russia were being prepared and U.S. President Joe Biden is expected to announce a new military aid package about the same size as last week's $800 million one in the coming days, sources told Reuters.

Russia has denied using banned weapons or targeting civilians and says, without evidence, that signs of atrocities were staged.

The Geneva-based U.N. High Commissioner for Refugees (UNHCR) on Wednesday put the total number of refugees at 5.01 million. More than half of the total, over 2.8 million, fled at least at first to Poland. Although many have stayed there, an unknown number have traveled onward. There are few border checks within the EU. In addition to the refugees, the U.N. says that more than 7 million people have been displaced within Ukraine.

Ukraine had a prewar population of 44 million.

The UNHCR said on March 30 that 4 million people had fled Ukraine and that the exodus was somewhat slower in recent weeks than at the beginning of the war.