Biden thanks US troops in Poland, praises 'guts' shown by Ukrainians

US President Joe Biden was a few dozen kilometres from the Ukrainian border on Friday to speak to US troops protecting NATO's eastern flank and to be briefed on Poland's response to the influx of refugees fleeing Russian attacks.

Biden also paid tribute to the "guts" shown by Ukrainians as the war grinds into its second month and the humanitarian crisis worsens.

"The Ukrainian people have a lot of backbone. They have a lot of guts. And I'm sure you are observing it. And I don't just mean the military ... but also the average citizen," he told a room of US service members in the small city of Rzeszow, some 65 kilometres from Ukraine.

"Look at how they are stepping up ... Women, young people, standing in front of a damn tank saying 'I'm not leaving.'"

Biden said there was a contest going on around the world between "democracies and oligarchs," a comparison he has frequently made since taking office.

Biden, who ate pizza with the soldiers, thanked them for their service, noting that there were now some 100,000 US troops stationed in Europe since recent reinforcements prompted by Russia's aggression.

The US transferred 4,700 soldiers from the 82nd Airborne Division from North Carolina to NATO member Poland just before the war broke out. These troops reinforced the 4,500 soldiers that Washington had already had stationed in Poland for some time. Despite this, there is no permanent US military base in Poland.

While in Rzeszow, Biden once again labelled Russian leader Vladimir Putin a "war criminal," a designation he first used on March 16.

The State Department this week said that, after conducting a review, the US has determined Russian troops have committed war crimes in Ukraine, citing the destruction of apartment buildings, schools, hospitals, shopping centres and critical infrastructure.

Biden later joined Polish President Andrzej Duda in Rzeszow to learn about the humanitarian efforts being made by Poland to support refugees.

Almost 2.24 million people have fled Ukraine to seek safety in Poland.

"Those little babies. Little children. You're looking at mothers - you don't have to understand the language they speak - you see in their eyes pain, and I mean literally pain, watching their children," Biden said,

"It's like something out of a science fiction movie," Biden said of the devastation in Ukraine. "You turn on the television and see what these towns look like, and the cities."

Biden said security concerns prevented him from getting closer to the border with Ukraine.

He travelled on to Warsaw later Friday. On Saturday he is due to meet refugees and deliver a speech from there on the war on Saturday.

Before Poland, Biden was in Brussels, where he attended a NATO summit, a meeting of the G7 group of major industrialized nations, and an EU summit. Among other things, Biden announced that the US would take in up to 100,000 refugees from Ukraine. He also confirmed that there would be further US sanctions on Russia to come.