As Russian troops have appeared to stall in their advance on Ukrainian cities, the United States voiced concern that China might assist Moscow with military equipment as the war entered its fourth week.
Ukraine's capital Kyiv came under renewed Russian shelling as rescuers in the besieged port of Mariupol dug survivors from the rubble of bombed buildings. Officials from the two countries met again for peace talks but said their positions remained far apart.
Western sources and Ukrainian officials said Russia's assault has faltered since its troops invaded on February 24, further dashing expectations of a swift victory and the removal of President Volodymyr Zelenskiy's government.
Despite battleground setbacks and punitive sanctions by the West, Russian President Vladimir Putin has shown little sign of relenting. His government says it is counting on China to help Russia withstand blows to its economy.
The United States, which this week announced $US800 million ($A1.1 billion) in new military aid to Kyiv, is concerned Beijing is "considering directly assisting Russia with military equipment to use in Ukraine", Secretary of State Antony Blinken said.
President Joe Biden will make clear to China's President Xi Jiping in a call Friday that Beijing "will bear responsibility for any actions it takes to support Russia's aggression, and we will not hesitate to impose costs", Blinken told reporters.
China has refused to condemn Russia's action in Ukraine or call it an invasion. It says it recognises Ukraine's sovereignty but Russia has legitimate security concerns that should be addressed.
While the United States says it wants to avoid direct confrontation with Russia, Chinese military aid to Moscow would pit Washington and Beijing on opposite sides of the conflict.
The war has settled into a grinding pattern of sieges of cities, with Ukrainian officials reporting Russian attacks on schools, hospitals and cultural facilities.
The United Nations has recorded 2032 civilian casualties in Ukraine - 780 killed and 1252 injured. Some 3.2 million civilians have fled to neighbouring countries.
A fourth straight day of talks between Russia and Ukrainia took place by video link, but the Kremlin said an agreement had yet to be reached.
Moscow has previously said it was close to agreeing to a formula that would keep Ukraine neutral, one of its demands.
Ukraine has said it is willing to negotiate an end to the war but will not surrender or accept Russian ultimatums.
Kyiv and its Western allies say Russia launched the war to subjugate a neighbour Putin calls an artificial state. Moscow says it is carrying out a "special operation" to disarm Ukraine.
Ukraine's Zelenskiy addressed the German Bundestag by video link on Thursday, giving a speech that invoked the Holocaust and the Berlin Wall, and seemed intended to shame pro-Russian politicians in Germany, 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."
The Russians have failed to capture a major city in the face of spirited resistance from Ukrainian forces protecting residential areas under daily bombardment.
Rescuers in Mariupol, a southern port city, dug survivors from the rubble of a theatre was hit by an air strike on Wednesday as civilians took shelter there from bombardments. The number of casualties is not yet known. Russia denies striking the theatre.
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.
Northern suburbs of Kyiv have suffered heavy damage but the capital itself has held firm.
The Ukrainian defence ministry said Russian forces had made no significant recent advances around Kyiv and had resorted to "chaotic" shelling.
British military intelligence said Russia's invasion has largely stalled, with Russian forces suffering heavy losses and making minimal progress on land, sea or air.
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