Moscow has launched dozens of air strikes across eastern Ukraine overnight, its defence ministry has said, as Russia's top diplomat acknowledged that "another phase" of the conflict was beginning as fighting raged in the Donbas region.

Ukrainian officials said their soldiers would withstand the assault, calling it the Battle of the Donbas. But the Russians pressed an advance across almost the entire stretch of the eastern front and, hours after its start, seized a frontline city.

In the ruins of Mariupol, the south-eastern port that has suffered nearly eight weeks of siege, Russia gave the last Ukrainian defenders holed up in a steel works an ultimatum to surrender by noon or die. The deadline passed without word of their condition.

Kyiv's lead negotiator said it was hard to predict when peace talks might resume because of the Mariupol siege and the new offensive.

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In a swift response to the intensified assault, US President Joe Biden and other Western leaders discussed increasing military, economic and humanitarian support for the Kyiv government, and ways to hold Moscow accountable, the White House said.

German Chancellor Olaf Scholz said Berlin intended to supply Ukraine with anti-tank and air defence weapons, as well as long-range artillery weapons, and Britain's prime minister, Boris Johnson, also promised more artillery weapons as the conflict moved into a new phase.

Mr Scholz said the allies agreed that Russia must not win the war, and that an imposed peace as envisaged by Russian President Vladimir Putin was not acceptable.

Rubble lies outside a kindergarten that was bombed during the Russian invasion west of Kyiv

The Italian government said there had been "broad consensus on the need to step up pressure on the Kremlin, including by adopting further sanctions, and to increase Moscow's international isolation".

UN Secretary-General Antonio Guterres called for a four-day humanitarian pause in the fighting over the Orthodox Easter weekend to allow civilians to leave areas of conflict and humanitarian aid to be delivered.

City captured

Ukraine said the new assault had resulted in the capture of Kreminna, an administrative centre of 18,000 people in Luhansk, one of the two Donbas provinces.

Russian forces were attacking "on all sides", authorities were trying to evacuate civilians and it was impossible to tally the civilian dead, Luhansk regional governor Serhiy Gaidai said.

President Volodymyr Zelensky told Ukrainians in a video address overnight: "No matter how many Russian troops they send there, we will fight. We will defend ourselves."

In Moscow, Russian Foreign Minister Sergei Lavrov confirmed that "another stage of this operation is beginning". Defence Minister Sergei Shoigu said Russia was "methodically" carrying out its plan to "liberate" Donetsk and Luhansk, provinces which Moscow demands Kyiv cede fully to Russian-backed separatists.

Driven back by Ukrainian forces in March from an assault on Kyiv in the north, Russia has instead poured troops into the east for the Donbas offensive. It has also made long-distance strikes at other targets including the capital.

Ukraine's top security official, Oleksiy Danilov, said Russian forces had tried to break through Ukrainian defences "along almost the entire front line of Donetsk, Luhansk and Kharkiv regions".

A woman looks at the coffin of her husband after the funeral at a cemetery in Bucha

The coal- and steel-producing Donbas has been the focal point of Russia's campaign to destabilise Ukraine since 2014, when the Kremlin used proxies to set up separatist "people's republics" in parts of Luhansk and Donetsk provinces.

Moscow now says its aim is to capture the full provinces on the separatists' behalf. Ukraine has a large force defending northern parts of the Donbas and military experts say Russia aims to cut them off or surround them.

However, Russia still needs to keep its troops supplied across miles of hostile territory.

Ukraine has counter-attacked near Kharkiv in the rear of Russia's advance, apparently aiming to cut off supply lines.

Since starting what it calls a special operation to demilitarise Ukraine on 24 February, Russia has bombed cities to rubble and hundreds of civilian bodies have been found in towns where its forces withdrew. It denies targeting civilians and says, without evidence, that signs of atrocities were staged.

Western countries and Ukraine accuse Putin of unprovoked aggression.

Shell-shocked residents

An elderly woman talks on a mobile phone in the village of Yampil

In Mariupol, scene of the war's heaviest fighting and worst humanitarian catastrophe, a last group of Ukrainian defenders holding out in the Azovstal steel works defied Russian calls to surrender and be spared.

However, about 120 civilians living next to the sprawling plant left via humanitarian corridors, the Interfax news agency has reported, quoting Russian state TV.

The pro-Kremlin leader of Chechnya, whose forces have been fighting in Mariupol, predicted troops would capture the plant.

Mariupol has been besieged since the war's early days. Tens of thousands of residents have been trapped with no access to food or water and bodies litter the streets. Ukraine believes more than 20,000 civilians have died there.

Capturing it would link pro-Russian separatist territory with the Crimea region that Moscow annexed in 2014.

In Russian-held districts reached by Reuters, shell-shocked residents cooked on open fires outside their damaged homes.

"Obviously, against the backdrop of the Mariupol tragedy, the negotiation process has become even more complicated," said Ukraine's lead negotiator Mykhailo Podolyak.

Kyiv and Moscow have not held face-to-face talks since 29 March. Each side blames the other for their breakdown.

"It is difficult to say when the next face-to-face round of negotiations will be possible because the Russians are seriously betting on the so-called 'second stage of the special operation'," Mr Podolyak said.

Shells and rockets also hit Kharkiv, Ukraine's second city, local officials said, wrecking apartment blocks and other buildings. Four people were killed and 14 wounded, they said.