WASHINGTON (AFP, REUTERS) - US President Joe Biden labelled Vladimir Putin a "war criminal" on Wednesday (March 16) after watching harrowing TV footage of the invasion of Ukraine, triggering an immediate angry riposte from Russia.

It was the first time Biden has used the phrase to describe Putin, and the US president then deepened his attack, tweeting that Putin was "inflicting appalling devastation and horror on Ukraine - bombing apartment buildings and maternity wards".

The term war criminal applies to anyone who violates a set of rules adopted by world leaders known as the law of armed conflict, according to The Associated Press. The rules govern how countries behave in times of war.

AP reported that the most likely way Putin could come into the picture as a war criminal is through the widely recognised legal doctrine of command responsibility. If commanders order or even know or are in a position to know about crimes and did nothing to prevent them, they can be held legally responsible.

Referring to reports that Russian troops took hostage 400 staff and patients in a Mariupol hospital, Biden said that "these are atrocities. It is an outrage to the world".

White House Press Secretary Jen Psaki said Biden was "speaking from his heart" after seeing images on television of "barbaric actions by a brutal dictator".

Putin's spokesman Dmitry Peskov told Russian news agencies that Biden's rhetorical attack was "unacceptable and unforgivable on the part of the head of a state, whose bombs have killed hundreds of thousands of people around the world".

Putin ordered a large-scale invasion of Ukraine three weeks ago, saying Russia wants to force the disarmament of Ukraine's military and topple the pro-Western government.

With the Russian advance largely stalled by Ukrainian troops, Moscow has turned increasingly to bombardments of civilians, prompting three million Ukrainians to become refugees.

Psaki said the US State Department already has "a legal process that is ongoing" to examine Russia's actions.

Ukrainian President Volodymyr Zelensky made an impassioned video address to US lawmakers earlier Wednesday, bolstered by a montage of horrific TV footage of Ukrainian civilians under Russian assault.

 He appealed for tougher sanctions on Russia and more weapons to help his country, repeating a request for a no-fly zone over Ukraine, something the West fears would worsen the conflict.

“In the darkest time for our country, for the whole of Europe, I call on you to do more,” said Zelensky, who showed video clips of dead and wounded children and blasted buildings.

At the request of Mr Zelensky, Washington will help Ukraine acquire additional long-range missile anti-aircraft systems as the West bolsters his government, which still hopes for a diplomatic solution.

Mr Zelensky said negotiations with Russia were becoming “more realistic” and Russia’s foreign minister said proposals under discussion were “close to an agreement”.

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

Footage from Ukraine’s state broadcaster showed bodies lying on the street. The Ukrainian general prosecutor’s office said it had opened an investigation.

Emergency services said rescue workers had found the bodies of five people, including three children, during searches of shell-hit residential buildings in Chernihiv.

In the besieged southern port of Mariupol, the city council said Russian forces bombed a theatre where civilians were sheltering. The number of casualties was not known.

Russia’s defence ministry denied it had carried out an air strike against the theatre. Reuters could not independently verify the information.

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.

Maxar Technologies, a private US company, distributed satellite imagery that it said was collected on March 14 and showed the word “children” in large Russian script painted on the ground outside the red-roofed Mariupol Drama Theatre.

The governor of Zaporizhzhia region said Russian forces had fired artillery at a convoy of evacuees from Mariupol, wounding five people on Wednesday. Ukraine’s military said children were among the casualties.

Three people were killed and five wounded after shelling caused a fire at a market in eastern Kharkiv, Ukraine’s second city, emergency services said.

Moscow denies targeting civilians.

Kyiv’s mayor Vitali Klitschko said Russian shelling caused a fire and damaged private homes and a gas line there on Wednesday evening after early morning barrages.

Russian troops have halted at the gates of the capital after taking heavy losses in a war Western officials say Moscow expected to win within days.

And as an information battle goes on alongside fighting, Facebook on Wednesday removed official Russian posts that falsely claimed reports of Russia bombing a children’s hospital in Ukraine were a hoax.

Russia has shifted its stance over the bombing that caused outrage around the world with a mix of statements that veered between aggressive denials and a call to establish clear facts. 

Alongside additional US help announced on Wednesday, North Atlantic Treaty Organisation (Nato) defence ministers meeting in Brussels also pledged more arms supplies to help Ukraine.

Negotiations were due to resume on Wednesday by a video link for a third straight day.

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.

Ukraine’s chief negotiator said Kyiv was still demanding a ceasefire and the withdrawal of Russian troops, with guarantees.

Zelensky has said Ukraine could accept international security guarantees that stopped short of its longstanding aim to join Nato, one of Russia’s main demands before it invaded.

“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.”

Kremlin spokesman Dmitry Peskov said the idea “could really be seen as a compromise”.

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

While Russia has long objected to Ukraine’s Nato ambitions, Kyiv and its allies have said Moscow’s true aim was to overthrow Ukraine’s pro-Western, elected leaders.

Ukrainian forces have withstood an assault by a much larger army. Zelensky said Ukrainian troops had killed a fourth Russian major general. Reuters was not able to verify this.

Ukraine said about 20,000 people had escaped besieged Mariupol in cars, but hundreds of thousands remain trapped under bombardment without heat, power or water. More than 3 million refugees have now fled Ukraine for other countries.

Russia was due to pay US$117 million (S$159 million) in interest on dollar-denominated sovereign bonds, but may pay in roubles instead, amounting to its first default on foreign debt since the Bolshevik revolution.

Moscow said it had the money, and Washington would be to blame if it cannot pay.

In his most explicit acknowledgment of the pain inflicted by Western sanctions, Putin said inflation and unemployment would rise, and structural changes to the economy would be needed.