KYIV/KHARKIV, April 19 (Reuters) - Russian troops seized a frontline Ukrainian city on Tuesday, hours after starting a long-anticipated offensive in eastern Ukraine that the Kyiv government has called the Battle of the Donbas.
Thousands of troops pressed an advance across almost the entire stretch of the eastern front as the assault began with massive Russian artillery and rocket barrages. Ukrainian officials said their soldiers would withstand the offensive.
In the ruins of Mariupol, the southeastern port that has withstood nearly eight weeks of siege, Russia gave the last Ukrainian defenders holed up in a steel works an ultimatum to surrender by noon (0900 GMT) or die.
Ukraine's lead negotiator meanwhile said it was hard to predict when peace talks might resume because of the Mariupol siege and the new military offensive.
On the international front, U.N. 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.
U.S. President Joe Biden consulted with fellow Western leaders "to discuss our continued support for Ukraine and efforts to hold Russia accountable", the White House said.
Ukraine said the new Russian 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 Zelenskiy told Ukrainians in a video address overnight that they would withstand the new advance.
"No matter how many Russian troops they send there, we will fight. We will defend ourselves," he said.
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.
CHANGE OF PLAN
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".
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.
After Russia's armoured assault fell prey early in the conflict to Ukrainian units armed with Western anti-tank missiles, Moscow may now hope a more conventional battle of armies in the Donbas will play to its firepower advantage.
But Russia still needs to keep its troops supplied across miles of hostile territory. For its part, Ukraine has counter-attacked near Kharkiv in the rear of Russia's advance, apparently aiming to cut off supply lines.
Since launching what it calls a special operation to demilitarise Ukraine on Feb. 24, 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 Russian President Vladimir Putin of unprovoked aggression.
SHELL-SHOCKED RESIDENTS
In Mariupol, scene of the war's heaviest fighting and worst humanitarian catastrophe, a last group of Ukrainian defenders defied Russian calls to surrender.
"All who lay down their arms are guaranteed to remain alive," the Russian defence ministry said.
The pro-Kremlin leader of Chechnya, whose forces have been fighting in Mariupol, predicted troops would capture the steel plant on Tuesday.
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," Ukraine's lead negotiator Mykhailo Podolyak told Reuters.
Kyiv and Moscow have not held face-to-face talks since March 29. 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'," Podolyak said.
In Kharkiv, Ukraine's second city, shells hit the southeastern Nemyshlianskyi district in the afternoon, wrecking one apartment building and damaging others.
Three bodies of people apparently killed by shrapnel lay on the pavement.
"They are sabotaging the whole city," said 79-year-old Fyodor Bondarenko, watching as one body was carried into an ambulance while the crump of shelling sounded in the background.
