The death toll from the bombing of a theatre where hundreds of people – including children – are said to have been sheltering is still unknown, Ukraine’s president has said.

Foreign ministry officials said civilians were buried under rubble after Wednesday’s attack in Mariupol and shelling was preventing them from assessing the number of casualties.

Days before the attack, a satellite image of the building shows the Russian word for ‘children’ written in giant letters at both ends of the building, in an apparent bid to protect it from the shelling.

Live updates – Putin rages over ‘unforgivable’ Biden remarks

Russia’s defence ministry denied targeting the attack, according to Russian news agency RIA, instead accusing a far-right Ukrainian militia of blowing it up – but provided no evidence for this claim.

“Russian aircraft purposefully dropped a huge bomb on the drama theatre in the city centre,” said President Zelenskyy in his nightly address.

“Hundreds of people were hiding from the shelling there… The building was destroyed. The death toll is still unknown.”

It comes as another morning attack on Kyiv, just after 5am on Thursday, killed at least one person and wounded three.

Emergency services said the 16-storey building was hit by remains of a downed missile.

Read more: Mariupol – a city under siege

In other developments:

• Zelenskyy to address German parliament this morning
• Putin urges “self-purification” to rid Russia of anyone who questions invasion
• Fourth Russian general killed in fighting, Ukraine says
• UN Security Council to hold meeting on Ukraine later today
• People in at least seven cities in Belarus report hearing explosions late Wednesday

There was also condemnation yesterday when 10 people lining up to buy bread were killed in Chernihiv, according to Ukrainian prosecutor’s office.

Russia denies deliberately targeting civilians, but such cases appear to have fuelled President Biden’s incendiary assertion that Vladimir Putin is a “war criminal”.

Dmitry Peskov, Mr Putin’s spokesman, called it “unacceptable and unforgivable rhetoric by the leader of a country from whose bombs hundreds of thousands of people have died”.

Another $800m of military aid to Ukraine has been approved after President Zelenskyy addressed the US Congress and invoked the 9/11 and Pearl Harbor attacks.

America’s additional pledge includes 800 Stinger anti-aircraft systems; 2,000 surface-to-air Javelin missiles; 6,000 portable anti-tank weapons and 100 so-called Switchblade “kamikaze” drones.

Despite peace talks appearing to make progress – with hints that Ukraine may agree to stay neutral and out of NATO – the situation on the ground remains grim.

An artillery shell started a fire and wiped out the top floor of another high-rise in Kyiv on Wednesday, with casualty numbers unclear.

Relentless strikes also pounded the second-biggest city of Kharkiv, in the northeast. However, officials said they were still managing to hold off Russian moves into the heart of the city.

Another apparent success was in occupied Kherson, on the Black Sea, where new satellite images of the airport appeared to show Russian helicopters and vehicles on fire after a strike on Tuesday.

Ukraine’s General Staff said the Russians were trying to remove any surviving military equipment, and that further afield its ground offensive on major cities had mostly stalled.

In Melitopol a mayor kidnapped five days ago has also now been freed, President Zelenskyy said.

Video last week had showed Ivan Fedorov being marched out of city hall, apparently surrounded by Russian personnel.

“I talked to him today. The Russian military abducted him on March 11, trying to persuade him to collaborate. But our man withstood. He did not give up,” said Mr Zelenskyy.

The Interfax Ukraine news agency – quoting the president’s press aide – said he was swapped for nine captured Russian soldiers.

The three-week war has now killed 726 civilians, the UK’s Ministry of Defence said in its latest update on Wednesday night, citing UN figures.

However, it said the true casualty figure “is likely to be significantly greater”.