As Kremlin forces step up attacks across Ukraine, US President Joe Biden has described Vladimir Putin as a butcher who “cannot remain in power” after meeting Ukrainian refugees in Poland.

Biden’s comments on Saturday, an escalation of US rhetoric towards Moscow over its invasion of Ukraine, were walked back within minutes by White House aides who said the President’s call for regime change in Russia did not actually mean that Putin should be ousted.

Instead, according to a White House official who briefed reporters, Mr Biden was calling for the world’s democracies to prepare for an extended conflict.

The President’s off-the-cuff remark and hasty White House effort to deny its clear meaning reflect US concern that a desperate Putin be given no excuse to expand his aggression beyond Ukraine.

Just before Mr Biden spoke outside Warsaw’s historic castle, four missiles hit the outskirts of Lviv, just 60km from the Polish border, local officials said.

Another strike significantly damaged Lviv’s infrastructure but caused no reported deaths.

Meanwhile, as the fight since Russia’s February 24 invasion of its neighbour drags on, a visibly irritated Ukrainian President Volodymyr Zelenskiy again demanded Western nations send military hardware.

He asked whether they were intimidated by Moscow, saying, “We’ve already been waiting 31 days.”

Biden’s fiery speech came at the end of a European trip aimed at bolstering Western resolve and framing the war as part of a historic struggle for democratic freedoms.

“For God’s sake, this man cannot remain in power,” said Biden, who branded Putin “a butcher”.

The Kremlin dismissed the comment, saying it was “not for Biden to decide” who Russians choose to elect.

“We need to be clear-eyed. This battle will not be won in days or months,” Mr Biden said. “We need to steel ourselves for a long fight ahead.”

Moscow says the goals for what Putin calls a “special military operation” include demilitarising and “denazifying” its neighbour. Ukraine and its Western allies calls this a pretext for an unprovoked invasion.

Russia has failed to seize any major Ukrainian city and the conflict has killed thousands of people, sent nearly 3.8 million abroad and driven more than half of Ukraine’s children from their homes, according to the United Nations.

US President Joe Biden and Polish President Andrzej Duda meet in Warsaw. Photo: Getty

Western intelligence officials say Russian forces now rely on indiscriminate bombardments rather than risking large-scale ground operations, a tactic that could limit Russian military casualties but would harm more civilians.

Russian forces seized Slavutych, a town where workers at the defunct Chernobyl nuclear plant live, and the mayor said three people were killed, Interfax Ukraine news agency said.

Ukrainian staff have continued to work at Chernobyl after the site of the world’s worst nuclear accident was seized by Russian forces. The International Atomic Energy Agency has expressed alarm about the situation.

Russian forces fired at a nuclear research facility in Kharkiv, Ukraine’s parliament said.

In the encircled southern port of Mariupol, Mayor Vadym Boichenko said the situation remained critical, with street fighting in the centre of the historic city. Mariupol has been devastated after weeks of pounding by Russian missiles and artillery.

The US, which has pledged billions in aid, promised an additional $US100 million ($A133 million) for field gear and civilian security assistance for Ukraine’s border guard and police.

President Zelenskiy compared Mariupol’s devastation to the destruction inflicted on the Syrian city of Aleppo by Syrian and Russian forces in Syria’s civil war.

He also warned of dire consequence if Ukraine – one of the world’s major grain producers – could not export its foodstuffs and urged energy-producing countries to boost output so Russia cannot use its oil and gas wealth to “blackmail” other nations.

The United Nations has confirmed 1104 civilian deaths and 1754 injuries in Ukraine but says the real toll is likely to be far higher. Ukraine says 136 children have been killed.

Russia’s defence ministry claims 1351 Russian soldiers had been killed and 3825 wounded, the Interfax news agency reported on Friday. Ukraine says 15,000 Russian soldiers have been killed.

Ukraine’s deputy prime minister, Iryna Vereshchuk, said an agreement had been reached to set up 10 humanitarian corridors on Saturday to evacuate civilians from frontline hotspots.

More than 100,000 people still needed to be evacuated from the Mariupol, Ms Vereshchuk said.

To the north, battle lines near the capital Kyiv have been frozen for weeks with two main Russian armoured columns stuck northwest and east of the city.

A British intelligence report said Russian forces were relying on indiscriminate air and artillery bombardments rather than risk large-scale ground operations.

-with AAP