KYIV: Ukraine's leader on Thursday accused Moscow of building a new Cold War wall across Europe "between freedom and bondage", as his government said Russian shelling had killed 21 more civilians.

Three weeks into their devastating invasion, Russian forces also stood accused of bombing a theatre sheltering many civilians and marked with the word "children".

Kyiv emerged from a 35-hour curfew to its own fresh scenes of destruction, as Russian troops strive to encircle the Ukrainian capital as part of their slow-moving offensive.

Beneath a Kyiv apartment block damaged by a downed rocket, AFP journalists saw a distraught man crouched over a body draped in a bloodstained cloth.

The 21 were killed when overnight artillery fire pounded a school and a cultural centre in the town of Merefa outside the hard-hit eastern city of Kharkiv, regional prosecutors said.

In besieged Mariupol to the south, from which officials say 30,000 civilians have now fled, rescuers were combing through the smoking rubble of the Drama Theatre.

Ukrainian officials said more than 1,000 civilians had been sheltering in a basement bomb shelter beneath the theatre, and that Russian shelling was continuing. Human Rights Watch said it was at least 500.

'Tear down this wall'

President Volodymyr Zelensky said the "number of dead is not yet known" at the theatre, but the airborne attack showed "Russia has become a terrorist state".

Zelensky addressed the German parliament a day after a speech to the US Congress, when he secured $1 billion in new US military aid, including Stinger anti-aircraft missiles like those used against Soviet forces in Afghanistan.

Zelensky reached back to that Cold War era as he drew on a 1987 speech in Berlin by US president Ronald Reagan: "Dear Mr Scholz, tear down this wall," he implored German Chancellor Olaf Scholz.

"It's not a Berlin Wall -- it is a wall in central Europe between freedom and bondage and this wall is growing bigger with every bomb."

In an overnight video message, Zelensky also urged Russians to lay down their arms and abandon an invasion that has drawn tough Western sanctions against President Vladimir Putin's regime.

"If your war, the war against the Ukrainian people, continues, Russia's mothers will lose more children than in the Afghan and Chechen wars combined," he said, referring to the thousands lost in those conflicts.

'It is hell'

US President Joe Biden called Putin a "war criminal", triggering fury in the Kremlin, as the Russian leader also lashed out at "scum and traitors" at home who he said were undermining the war effort.

Without offering evidence, Russia's defence ministry accused Ukraine's far-right Azov Battalion of blowing up the theatre in Mariupol.

Local officials say more than 2,000 people have died so far in indiscriminate Chechnya-style shelling of the port city.

People fleeing Mariupol said they were forced to melt snow for drinking water and cook food scraps on open fires, with water and power supplies cut off.

"In the streets there are the bodies of many dead civilians," Tamara Kavunenko, 58, told AFP after reaching the central city of Zaporizhzhia.

"It's not Mariupol anymore," she said. "It is hell."

Satellite images of the Drama Theatre on March 14 shared by private satellite company Maxar showed the word "children" clearly written in Russian on the pavement on either side of the building.

Officials posted a photo of the building, its middle part completely destroyed and thick white smoke rising from the ruins.

Mariupol mayor Vadim Boychenko described the strike on the theatre as an act of "genocide".

The Group of Seven most industrialised nations Thursday warned that perpetrators of war crimes in Ukraine will be "held responsible".

Escaping default

Addressing the Bundestag by video, Zelensky issued a strong rebuke of Germany's years-long reluctance to sever energy and business ties with Russia.

"We turned to you," he said. "We told you that Nord Stream (gas pipeline) was a kind of preparation for the war.

"And the answer we got was purely economic -- it is economy, economy, economy but that was the mortar for the new wall."

The broader economic consequences from the war could cut global growth by "over one percentage point" in the next 12 months, the Organisation for Economic Cooperation and Development said.

But NATO members have resisted Zelensky's pleas for direct involvement through a no-fly zone over Ukraine, warning it could lead to World War III against nuclear-armed Russia.

Putin, at a televised government meeting Wednesday, insisted the invasion was "developing successfully", adding: "We will not allow Ukraine to serve as a springboard for aggressive actions against Russia."

He also condemned the Western sanctions as "economic blitzkrieg", after Russia was frozen out of much of the Western financial system.

But Russia's finance ministry said Thursday it had made interest payments worth $117.2 million on two foreign bonds, avoiding a default for now.

From rackets to rifles

More than three million Ukrainians have fled across the border, mostly women and children, according to the UN.

With stop-start peace talks ongoing, officials in Kyiv said Russia had agreed to nine humanitarian corridors Thursday for fleeing refugees, including one out of Mariupol.

Kremlin spokesman Dmitry Peskov had said a "compromise" outcome in the talks would centre on Ukraine becoming a neutral state comparable to Sweden and Austria -- an idea roundly rejected by Kyiv.

French Foreign Minister Jean-Yves Le Drian told Le Parisien newspaper that Russia persisted with "maximalist demands -- wanting Ukraine to surrender and intensifying siege warfare".

Russia was reprising the brutal tactics it used in Chechnya and Syria, he said, and resorting to "talks with no objective other than pretending that they are negotiating".

Pro-Kremlin Chechen strongman Ramzan Kadyrov said 1,000 fighters led by one of his relatives were en route to Ukraine "to take part in the special operation of denazification and demilitarization of Ukraine".

Ukraine has asked Turkey to be a guarantor of any future deal with Russia, along with the UN Security Council's five permanent members and Germany, Turkish Foreign Minister Mevlut Cavusoglu said.

But for many Ukrainians, any talk of peace rings hollow.

Retired tennis player Sergiy Stakhovsky, who knocked Roger Federer out of Wimbledon in 2013, has traded his racquet for a rifle and returned to Kyiv.

His wife was distraught at the decision, Stakhovsky told AFP as he patrolled the city in khaki camouflage, toting a Kalashnikov. But, he said, "I knew I had to go there."