Bloody new attacks on civilians fuelled accusations Thursday that Russia is committing war crimes in Ukraine, as the United States warned it will make China pay for any support given to Moscow's assault.

Three weeks into Russia's devastating invasion, the harsh tally of assaults on civilian targets grew to include a school and a cultural centre in the town of Merefa, pounded by overnight artillery fire with 21 people killed, authorities said.

Despite the mounting carnage, punishing international sanctions and unexpectedly strong resistance from Ukrainians, top US diplomat Antony Blinken said Thursday he saw no sign that Russian leader Vladimir Putin "is prepared to stop."

"Intentionally targeting civilians is a war crime. After all the destruction of the past few weeks, I find it difficult to conclude that the Russians are doing otherwise," he said, following warnings from the G7 that those behind such crimes "will be held responsible."

Blinken was doubling down on the tough language used by President Joe Biden -- who a day earlier branded Putin a "war criminal" and on Thursday called him both a "thug" and a "murderous dictator."

He spoke as authorities were still trying to count the dead at the theatre targeted by a bomb in southern Mariupol a day earlier -- which Ukraine's leader Volodymr Zelensky held up as evidence "Russia has become a terrorist state".

In the latest of a series of resonant speeches to Western lawmakers, Zelensky told the German parliament that Moscow was building a new Cold War wall across Europe, "between freedom and bondage." 

Russia's unrelenting onslaught on Mariupol has drawn particular horror.

Local officials say more than 2,000 people have died so far in indiscriminate Chechnya-style shelling of the strategic port, and 80 percent of its housing has been destroyed.

Under new Russian shelling, rescuers were combing through the smoking rubble of the Drama Theatre, where Ukrainian officials said more than 1,000 civilians were sheltering in a basement bomb shelter when it was bombed. Human Rights Watch believes they numbered at least 500.

Among the 30,000 civilians said to have fled Mariupol so far, evacuees 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."

- US puts China on notice -
Biden and NATO have refused to get directly involved in the conflict, fearing an escalation with nuclear-armed Russia that would trigger World War III.

Instead Biden has successfully marshalled a tight Western alliance against Moscow, piling sanctions on Putin's regime while giving military support to Ukrainian forces.

But one potentially dangerous outlier looms: China.

Beijing has refused to condemn Moscow, and Washington fears the Chinese could switch to full financial and even military support for Russia, transforming an already explosive transatlantic standoff into a global dispute.

A phone call between Biden and his Chinese counterpart Xi Jinping is planned for Friday, and Blinken said his boss would press Beijing to help end the war, rather than support its fellow authoritarian ally.

Biden "will make clear that China will bear responsibility for any actions it takes to support Russia's aggression and we will not hesitate to impose costs," Blinken said.

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

- 'Tear down this wall' -
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.

But Blinken said that Moscow was not sincere, stating: "I have not seen any meaningful efforts by Russia to bring this war that it is perpetrating to a conclusion through diplomacy."

As the death toll mounts and Russian forces squeeze Kyiv, Zelensky continued his increasingly desperate pleas for more help, particularly for military hardware and a no-fly zone.

He has been making a series of impassioned speeches to lawmakers in Western nations, each one tailor-made by invoking the most stirring moments of their recent histories.

This time, before the German parliament, 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 urged Russians directly to lay down their arms.

"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.

- 'It is hell' -
The Ukrainian leader was speaking virtually from Kyiv, which Russian troops are still trying to surround in a slow-moving offensive.

Fresh fighting broke out on the city's edge as Ukrainian and Russian forces traded shell and rocket fire in the northwest, AFP journalists witnessed.

Civilians ran for cover as shelling set fire to a building near a warehouse, across the road from a mall with a multiplex cinema.

Inside the warehouse's car park, a Ukrainian soldier carrying a rifle ran in a crouch as gunshots crackled.

A man carried a prone child in his arms into a nearby block of flats, and at least five ambulances raced towards the scene.

In Odessa, on the Black Sea, civilians were bracing for attack, with tanks deployed at intersections and monuments covered in sandbags. 

"Our beautiful Odessa," said Lyudmila, an elegant elderly woman wearing bright lipstick, as she looked apologetically at her city's empty, barricaded streets.

"But thank God we are holding on! Everyone is holding on!"

And elsewhere, civilians in bomb shelters did what they could to help forget the war raging outside -- even just for a while.

From her shelter in Ukraine's second city Kharkiv, Vera Lytovchenko has become a social media sensation with her violin performances and used the attention to launch a fundraiser.

"I want to help my friends and music teachers who have lost their homes, their jobs, their instruments," said the soloist.

"I don't want to feel helpless," she said.

Latest developments

Here are the latest developments in the war in Ukraine:

- Zelensky warns Berlin of new 'Wall' -
A day after pleading with US Congress to send more help to Ukraine, President Volodymyr Zelensky warns German lawmakers that Russia is building a new "Wall" in Europe, evoking the Berlin Wall during the Cold War.

Zelensky has been on a virtual tour of Western parliaments, each time receiving a standing ovation from MPs for his war-time leadership.

- 21 killed near Kharkiv -
Russian forces shell a school and cultural centre, killing at least 21 people and wounding 25 in the town of Merefa outside Kharkiv, in eastern Ukraine, local prosecutors say.

- Mariupol theatre attack -
Authorities in Ukraine's Mariupol say they are clarifying information on possible victims of the Russian shelling of a theatre sheltering civilians a day earlier.

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.

- 30,000 flee Mariupol -
The Mariupol authorities say around 30,000 people have fled the besieged port city, adding that "80 percent of residential housing was destroyed".

- Biden-Xi talks -
US Secretary of State Anthony Blinken says President Joe Biden will warn Chinese counterpart Xi Jinping when he speaks to him on Friday of retaliation if Beijing provides military aid to Russia.

- ExoMars mission suspended -
A Russian-European mission to land a rover on Mars has been suspended due to the sanctions over Moscow's invasion of Ukraine and its "tragic consequences", the European Space Agency says.

- War hits global growth -
The fallout from the war in Ukraine could cut global growth by over one percentage point over the coming year, the Organization for Economic Cooperation and Development says.

- No default -
Russia's finance ministry says it has made interest payments on two foreign bonds, quashing fears of a debt default as the country reels from unprecedented Western sanctions.

- Kyiv rocket kills one -
One person is killed and three injured when a downed rocket hits an apartment block, emergency services say, bringing to at least seven the number killed in Kyiv since Monday. 

The capital, which emerged from a 35-hour curfew, is hemmed in by Russian forces striving to encircle it as part of their slow-moving offensive.

- Peace talks -
At ongoing peace talks, officials in Kyiv say Russia has agreed to nine humanitarian corridors for fleeing refugees, including one out of Mariupol.

- Ukraine calls on Turkey -
Ukraine's Foreign Minister Dmytro Kuleba 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, Ankara's top diplomat says.

- 'War criminal' -
US President Joe Biden brands Russian President Vladimir Putin a "war criminal" for the first time. The Kremlin quickly punches back, calling the comment "unacceptable and unforgivable".

Blinken accuses Russia of committing war crimes by attacking civilians in Ukraine.

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

- Chechen strongman sends volunteers -
A thousand Chechen volunteers are on their way "to take part in the special operation of denazification and demilitarisation of Ukraine" Chechen strongman Ramzan Kadyrov says. 

- Stakhovsky takes up arms -
Retired Ukrainian tennis player Sergiy Stakhovsky, who in 2013 knocked defending champion Roger Federer out of Wimbledon, exchanges his racket for a rifle, vowing to defend Kyiv "to the end" against Russian forces.