KYIV/LVIV: Rescue workers dug survivors on Thursday out of the rubble of a theatre in the besieged city of Mariupol which Ukraine said had been hit by a Russian air strike while people sheltered there from bombardments.

Russia denied striking the theatre. But its forces have blasted cities and killed many civilians in its assault on Ukraine, now entering its fourth week.

Mariupol has suffered the worst humanitarian catastrophe of the war, with hundreds of thousands of civilians trapped in basements with no food, water or power as Russian forces pound it with artillery fire and air strikes.

A city mayoral adviser, Petro Andrushchenko, said the number of victims of the purported strike on the theatre on Wednesday was not known, but the shelter had held.

"Now the rubble is being cleared," he told Reuters by phone. "There are survivors."

Commercial satellite pictures showed the word "children" had been marked out on the ground in front of the building before it was hit.

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Russian foreign ministry spokeswoman Maria Zakharova said the allegation that Russia had bombed the theatre was a "lie", and repeated Kremlin denials that Russian forces have targeted civilians.

"Russia's armed forces don't bomb towns and cities," she told a briefing.

Mariupol city council later said more than 350,000 people were still sheltering in the city and 30,000 had left.

The assault on Ukraine started with troops crossing the border or landing by sea and air on Feb. 24. But Western countries say its expectations of a swift victory and the removal of President Volodymyr Zelenskiy's government have been dashed and its invasion force has got bogged down.

Sides far apart

The war has settled into a grinding pattern of sieges of cities. The United Nations said 3.2 million civilians, mostly women and children, have now fled Ukraine to neighbouring countries. But the Russians have failed to capture a major city in the face of spirited resistance from Ukrainian forces.

British military intelligence said in an update on Thursday that the invasion had "largely stalled on all fronts", and Russian forces were suffering heavy losses.

A fourth straight day of talks between Russian and Ukrainian negotiators took place by videolink but Western officials said the two sides remained far apart.

"Both sides are taking them (the talks) seriously but there is a very, very big gap between the positions in question," one official said.

An aide to Zelenskiy said Ukraine was sticking to its core position that it retain sovereignty over areas occupied since 2014 by Russian and pro-Russian forces.

Russian President Vladimir Putin has shown little sign of relenting, even though punitive Western sanctions are damaging his country's economy.

In a televised speech, he inveighed against "traitors and scum" at home who helped the West, and said the Russian people would spit them out like gnats.

Kyiv and its Western allies say Russia launched the war to subjugate a neighbour Putin calls an artificial state. Moscow says it is carrying out a "special operation" to disarm Ukraine.

On the diplomatic front, the White House said U.S. President Joe Biden would hold a call on Friday with Chinese leader Xi Jinping as Washington tries to persuade Beijing not to provide support to Russia.

Ukraine's Zelenskiy addressed the German Bundestag by video link, pulling no punches in a speech that invoked the Holocaust and the Berlin Wall, and seemed intended to shame pro-Russian politicians in Germany, Moscow's main energy buyer.

Russia invades from three sides

"Every year politicians repeat 'never again'," said Zelenskiy, who is of Jewish heritage, citing a slogan used to mark the Holocaust.

"And now we see that these words are simply worthless. In Europe a people is being destroyed, they are trying to destroy everything that is dear to us, what we live for."

He accused Germany of helping to build a new wall "in the middle of Europe between freedom and unfreedom", by isolating Ukraine with its business ties to Russia and its earlier support for Nord Stream 2, a gas pipeline it has since frozen.

Bloody sheet

Northeastern and northwestern suburbs of Kyiv have suffered heavy damage but the capital itself has held firm, under a curfew and subjected to deadly nightly rocket attacks.

A building in Kyiv's Darnytsky district was extensively damaged by what the authorities said was debris from a missile shot down early in the morning.

As residents cleared glass and carried bags of possessions away, a man knelt weeping by the body of a woman which lay close to a doorway, covered in a bloody sheet.

Viacheslav Chaus, governor of the region centred on the frontline northern city of Chernihiv, said 53 civilians had been killed there in the past 24 hours. The toll could not be independently verified.

Russia has assaulted Ukraine from four directions, sending two columns towards Kyiv from the northwest and northeast, pushing in from the east near the second biggest city Kharkiv, and spreading in the south from Crimea.

Ukrainian officials have said they think Russia is running out of troops to keep fighting and could soon come to terms with its failure to topple the Ukrainian government.

Moscow has said it is close to agreeing a formula that would keep Ukraine neutral, long one of its demands.