Russian forces in Ukraine are blasting cities and killing civilians but no longer making progress on the ground, Western countries said on Thursday, as a war Moscow was thought to have hoped to win within days entered its fourth week.

Local officials said rescuers in the besieged southern port of Mariupol were combing the rubble of a theatre where women and children had been sheltering, bombed by Russian forces the previous day.

"The bomb shelter held. Now the rubble is being cleared. There are survivors. We don’t know about the (number of) victims yet," mayoral adviser Petro Andrushchenko told Reuters by phone.

Russia denied striking the theatre, which commercial satellite pictures showed had the word "children" marked out on the ground in front before it was blown up.

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 for weeks. Russian forces have begun letting some people out in private cars this week but have blocked aid convoys from reaching the city.

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.

In the capital Kyiv, a building in the 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.

Although both sides have pointed to limited progress in peace talks this week, President Vladimir Putin, who ordered Russia's invasion on Feb. 24, showed little sign of relenting.

In a vituperative 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.

Dmitry Medvedev, deputy head of Putin's security council, said the United States had stoked "disgusting" Russophobia in an attempt to force Russia to its knees: "It will not work - Russia has the might to put all of our brash enemies in their place."

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

Heavily outnumbered Ukrainian forces have prevented Moscow from capturing any of Ukraine's biggest cities so far despite the largest assault on a European state since World War Two. More than 3 million Ukrainians have fled and thousands of civilians and combatants have died.

'Stalled on all fronts'

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 from a staunch and well-coordinated Ukrainian resistance.

 READ MORE

• Russia accuses Ukraine of trying to frame it over Mariupol theatre attack
• Russia, Ukraine war: US President Joe Biden calling Vladimir Putin 'war criminal' is 'unforgivable', Kremlin says
• 9yo Ukrainian girl hopes Russians 'didn't mean to hurt me' after losing arm and father in attack
• Russian embassy's Ukraine posts spark call for ambassador's expulsion

Ukraine's President Volodymyr 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 Moscow's main energy buyer.

"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.

Russia has assaulted Ukraine from four directions, sending two massive 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.

Northeastern and northwestern suburbs of Kyiv have been reduced to rubble by heavy fighting, but the capital itself has held firm, under a curfew and subjected to deadly nightly rocket attacks.

Amid the unrelenting fighting, both sides have spoken of progress at talks. 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.

Moscow said peace talks resumed on Thursday by videolink for a fourth straight day, discussing military, political and humanitarian issues. A top aide to Zelenskiy said Ukraine still maintained its core demand, that it retain sovereignty over areas occupied since 2014 by Russian and pro-Russian forces.