Russia condemns Biden's comments on Putin, calls it 'unforgivable'

US President Joe Biden called Russian leader Vladimir Putin a war criminal in comments the Kremlin said were "unforgivable" as it insisted the war in Ukraine was "going to plan" amid talk of compromise at peace talks.

Moscow has yet to capture any of Ukraine's biggest cities despite the largest assault on a European state since World War II. More than 3 million Ukrainians have fled and thousands have died as the war enters its fourth week.

Ukraine's foreign ministry said Russian forces dropped a powerful bomb on a theatre in Mariupol, leaving many civilians trapped and an unknown number of casualties. Reuters could not independently verify the information.

The theatre had been housing at least 500 civilians, according to Human Rights Watch.

Moscow denies targetting civilians and Russia's defence ministry said its forces had not struck the building, RIA news agency said. 

Thirteen buses carrying around 300 refugees from Mariupol have arrived in Russia's Rostov region, Russia's Interfax news agency cited the Russian defence ministry as saying on Thursday.

The US Embassy in Kyiv said Russian forces had shot dead 10 people waiting in line for bread in Chernihiv, northeast of Kyiv. Russia denied the attack and said the incident was a hoax.

The United Nations' top court for disputes between states ordered Russia on Wednesday to immediately halt its military operations in Ukraine, saying it was "profoundly concerned" by Moscow's use of force. 

'NEUTRAL STATUS'

Ukraine's armed forces are conducting small-scale counter attacks on several fronts and Russian troops have not been able to gain ground because of a lack of resources, Ukrainian presidential adviser Oleksiy Arestovych said on Wednesday.

There were signs of compromise and progress at ongoing talks between Russia and Ukraine.

The Kremlin said negotiators were discussing a status for Ukraine similar to that of Austria or Sweden, both members of the European Union that are outside the NATO military alliance.

"Neutral status is now being seriously discussed along, of course, with security guarantees," Russian Foreign Minister Sergei Lavrov said. "There are absolutely specific formulations which in my view are close to agreement."

Vladimir Medinsky, Russia's chief negotiator, told state TV: "Ukraine is offering an Austrian or Swedish version of a neutral demilitarised state, but at the same time a state with its own army and navy."

Austria and Sweden, the biggest of six EU members outside NATO, both have small militaries that cooperate with the alliance.

President Volodymyr Zelenskiy has said Ukraine could accept international security guarantees that stopped short of its longstanding aim to join NATO. That prospect has been one of Russia's primary concerns.

"My priorities during the negotiations are absolutely clear: the end of the war, guarantees of security, sovereignty, restoration of territorial integrity, real guarantees for our country, real protection for our country," Zelenskiy said in a video address released early on Thursday.

In a speech on Wednesday to the US Congress by video link, Zelenskiy repeated a request for a no-fly zone over Ukraine, something the West won't introduce.

"In the darkest time for our country, for the whole of Europe, I call on you to do more," he said.

The US announced an additional $800 million in security assistance to Ukraine, with the new package including drones, anti-armour and anti-aircraft systems.

"More will be coming as we source additional stocks of equipment that ... we are ready to transfer," Biden said, later condemning Putin.

NATO defence ministers meeting in Brussels also pledged more arms supplies to help Ukraine.

The UN Security Council is due to vote on Friday on a Russian-drafted call for aid access and civilian protection, but diplomats say the measure is set to fail because it does not push for an end to the fighting or withdrawal of troops.