Russian forces pressed their offensive in Ukraine on Wednesday, raining artillery and missile strikes along the long eastern front, as Ukrainian soldiers holding out inside a sprawling steel factory in the southeastern city of Mariupol warned that they could be killed within hours.

Even as the Ukrainian troops in Mariupol defied another Russian deadline to surrender, a tentative deal was reached to allow women and children to evacuate the besieged port city, although it was unclear whether civilians also sheltering inside the steel plant would be able to leave along the proposed departure route.

Along the 300-mile eastern front, which stretches from Mariupol north to Kharkiv, US and Ukrainian officials said that Russia continued to pour in soldiers, artillery units and military hardware as it launches a renewed assault aimed at grinding down and encircling Ukrainian forces, and capturing all of the eastern territory known as the Donbas.

“Virtually the entire combat-ready part of the invaders’ army is concentrated on the territory of our state and in the border areas of Russia,” Ukrainian President Volodymyr Zelensky said overnight.

As Russia refocuses on the east following its failure to take Ukraine’s capital, Kyiv, it is confronting Ukrainian forces that have spent years digging trenches and fortifying defensive positions during a grinding conflict with Russian-backed separatists that began in 2014 that resulted in the breakaway republics of Donetsk and Luhansk.

Although no major battles have yet been waged in the days since Russia signalled the start of the offensive, and no significant territory has changed hands, Moscow’s superior weaponry and a more favourable terrain suggest a bloody fight to come.

The United Nations said on Wednesday the number of refugees who have fled Ukraine since Russia invaded on February 24 had exceeded five million. More than half are children.

Ukraine said it had so far held off an assault by thousands of Russian troops attempting to advance in what Ukrainian officials call the Battle of the Donbas, a new campaign to seize two eastern provinces Moscow claims on behalf of separatists.

Russia’s nearly eight-week-long 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.

In the ruins of Mariupol, 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. Ukrainian officials have said women and children are trapped in bunkers under the plant.

“The world watches the murder of children online and remains silent,” 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.

Russian-backed separatists said shortly before a 1100 GMT) Wednesday deadline that just five people had surrendered.

New York Times News Service and Reuters