Ukrainian authorities are struggling to determine the fate of hundreds of civilians who had been sheltering in a theatre ripped apart by a Russian air strike in the besieged city of Mariupol.

A photo released by Mariupol’s city council showed an entire section of the large, three-storey theatre had collapsed after the strike on Wednesday evening.

Several hundred people had taken refuge in the building, seeking safety amid Russia’s three-week siege of the strategic Azov Sea port city.

At least as recently as Monday, the pavement in front of and behind the once-elegant theatre was marked with huge white letters spelling out “children” in Russian, according to images released by the Maxar space technology company.

Rubble had buried the entrance to the shelter inside the theatre, and it was not yet known how many people were killed or injured, said Pavlo Kyrylenko, head of the Donetsk regional administration.

Russian air strikes also hit a municipal swimming pool complex in Mariupol where civilians, including women and children, had been sheltering, Mr Kyrylenko said on Telegram.

“Now there are pregnant women and women with children under the rubble there,” he said.

“My heart breaks from what Russia is doing to our people,” Ukrainian President Volodymyr Zelensky said on Wednesday night after the theatre strike, and just hours after he delivered a speech via video to the US Congress that garnered several ovations.

The Russian defence ministry denied bombing the theatre or anywhere else in Mariupol on Wednesday.

On Thursday, Russian artillery destroyed a school and a community centre in Merefa, a city near the north-east city of Kharkiv, according to Merefa’s mayor Veniamin Sitov.

There were no known civilian casualties. The Kharkiv region has seen heavy bombardment as stalled Russian forces try to advance in the area.

Six nations have called for a UN Security Council meeting on Ukraine on Thursday afternoon, ahead of an expected Friday vote on a resolution demanding protection for Ukrainian civilians “in vulnerable situations”.

“Russia is committing war crimes and targeting civilians,” Britain’s UN Mission tweeted, announcing the call for the meeting that was joined by the US, France and others. “Russia’s illegal war on Ukraine is a threat to us all.”

Russian attacks have battered cities and villages across large parts of Ukraine, including the capital, Kyiv, where residents have been huddling in homes and shelters.

Russian troops shelled areas in and around the city on Wednesday, including a residential neighbourhood just 1.5 miles (2.5 kilometres) from the presidential palace. A 12-storey Kyiv apartment building erupted in flames after being hit by shrapnel.

It comes as Russian President Vladimir Putin called for a “self-purification” to rid his country of anyone who questions his invasion.

Mr Putin went on television to excoriate Russians who do not back him, even as both sides expressed optimism over efforts to negotiate an end to the fighting.

Russians “will always be able to distinguish true patriots from scum and traitors and will simply spit them out like a gnat that accidentally flew into their mouths,” he said.

“I am convinced that such a natural and necessary self-purification of society will only strengthen our country.”

He said the West is using a “fifth column” of traitorous Russians to create civil unrest.

“And there is only one goal, I have already spoken about it — the destruction of Russia,” he said.

The speech appeared to be a warning that his authoritarian rule, which had already grown tighter since the invasion began on February 24, shutting down Russian news outlets and arresting protesters, could grow even more repressive.

But it came amid signs that talks were finally making progress.

Russian foreign minister Sergei Lavrov said after Tuesday’s meeting that a neutral military status for Ukraine was being “seriously discussed” by the two sides, while Mr Zelensky said Russia’s demands for ending the war were becoming “more realistic”.

Wednesday’s talks, held by video, appeared to wade more deeply into technicalities.

Mr Zelensky’s adviser Mikhailo Podolyak said Ukraine demanded a ceasefire, the withdrawal of Russian troops and security guarantees for Ukraine from several countries.

“This is possible only through direct dialogue” between Mr Zelensky and Mr Putin, he tweeted.

An official in Mr Zelensky’s office told The Associated Press that the main subject under discussion was whether Russian troops would remain in separatist regions in eastern Ukraine after the war and where the borders would be.

The official said Ukraine was insisting on the inclusion of one or more Western nuclear powers in the negotiations and on a legally binding document with security guarantees for Ukraine. In exchange, the official said, Ukraine was ready to discuss a neutral status.

Russia has demanded that Nato pledge never to admit Ukraine to the alliance or station forces there.

Earlier on Wednesday, Mr Zelensky went before the US Congress via video and, invoking Pearl Harbour and 9/11, pleaded with America for more weapons and tougher sanctions against Russia, saying: “We need you right now.”

US President Joe Biden announced the US was sending an additional 800 million dollars (£608 million) in military aid to Ukraine. He also called Mr Putin a “war criminal”, in his sharpest condemnation since the invasion began.

Although Moscow’s ground advance on the Ukrainian capital appeared largely stalled, Mr Putin said earlier that the operation was unfolding “successfully, in strict accordance with pre-approved plans”. He also decried Western sanctions against Moscow, accusing the West of trying to “squeeze us, to put pressure on us, to turn us into a weak, dependent country”.

The fighting has led more than three million people to flee Ukraine, the UN estimates. The death toll remains unknown, though Ukraine has said thousands of civilians have died.

Meanwhile, Russian forces have freed the mayor of the Ukrainian city of Melitopol in exchange for nine of their captured conscripts, an official from Ukraine’s presidential office said.

Kyiv accused the Russians of kidnapping mayor Ivan Fedorov about a week ago. Surveillance video showed him being marched out of city hall apparently surrounded by Russian soldiers.

Residents of Melitopol, a city in the south-east currently under Russian control, have been protesting to demand his release.

Moscow initially denied sending conscripts to fight in Ukraine, but later the Russian military admitted that some conscripts have been involved in the offensive and even got captured by Ukrainian forces.