US President Joe Biden has called his Russian counterpart Vladimir Putin a "war criminal", prompting an angry response from the Kremlin which denounced it as "unforgivable rhetoric" from a head of state.

Biden's off-the-cuff remarks on Wednesday was the harshest condemnation of Putin's actions from any US official since Russia launched a 'special military operation' in Ukraine three weeks ago.

Previously, President Biden had stopped short of labelling atrocities being documented on the ground in Ukraine as "war crimes," citing ongoing international and US investigations.

White House Press Secretary Jen Psaki later defended Biden and said the president had been speaking from his heart after seeing "barbaric" images of the violence in Ukraine.

"I think the President's remarks speak for themselves. He was speaking from his heart and speaking from what we've seen on television, which are barbaric actions by a brutal dictator through his invasion of a foreign country," she said.

She also noted that there was a separate legal process, run by the State Department, to determine war crimes - and that was ongoing separately.

In Moscow, Kremlin described Biden's "war criminal" remarks as "unforgiveable rhetoric".

"We consider such rhetoric from a head of the state, whose bombs killed hundreds of thousands of people around the world, to be unacceptable and unforgivable,” Kremlin spokesman Dmitry Peskov was quoted as saying by Russian state news agency TASS.

Earlier on Wednesday, Biden announced another USD 800 million security assistance to Ukraine, taking the total to USD 1 billion in less than a week.

Biden's announcement came hours after Ukraine President Volodymyr Zelenskyy pleaded for more military aid in a historic address to the US Congress.

Zelensky urged the US politicians to remember their country coming under attack in the past - at Pearl Harbour in 1941 and on 11 September 2001 - saying Ukrainians were experiencing the same thing every day.

"In your great history, you have pages that would allow you to understand the Ukrainian history. Understand us now," he said.

Zelensky also urged American lawmakers and Biden for more help defending itself, including a no-fly zone and fighter jets.

President Biden also spoke about the "appalling devastation" inflicted on Ukraine, including reports of doctors and patients being held hostage in Mariopol.

"These are atrocities. They're an outrage to the world, and the world is united in our support for Ukraine and our determination to make Putin pay a very heavy price," Biden said.

The United States, its allies and partners, he said, are crippling Putin's economy with punishing sanctions.

"That's going to only grow more painful over time with the entire NATO and EU behind us, and many other countries. What's at stake here are the principles that the United States and the United Nations and across the world stand for. It's about freedom," Biden said.

"It's about the right of people to determine their own future. It's about making sure Ukraine will never be a victory for Putin, no matter what advances he makes on the battlefield.

"The American people are answering President Zelenskyy's call for more help, more weapons for Ukraine to defend itself, more tools to fight Russian aggression, and that's what we're doing," he said.

The new security package to Ukraine includes 800 anti-aircraft systems to make sure the Ukrainian military can continue to stop the Russian planes and helicopters that have been attacking their people and to defend their Ukrainian airspace, he said.

“We are not doing this alone. Our allies and partners have stepped up to provide significant shipments of security assistance and will continue to help facilitate these deliveries as well,” the US president said.