Joe Biden on Saturday in Warsaw met two Ukrainian ministers in the first face-to-face talks between the US president and top Kyiv officials since Russia's invasion began.

Foreign Minister Dmytro Kuleba and Defence Minister Oleksii Reznikov made a rare trip out of Ukraine in a possible sign of growing confidence in the fightback against Russian forces.

The meeting took place at the Marriott Hotel in the city centre -- opposite a Warsaw train station where there has been a constant flow of Ukrainian refugees since the conflict started.

Biden could be seen seated at a long white table between US Secretary of State Antony Blinken and US Defense Secretary Lloyd Austin, facing Kuleba and Reznikov, an AFP reporter said.

There were Ukrainian and US flags in the background.

The officials discussed "the United States' unwavering commitment to Ukraine's sovereignty and territorial integrity," State Department spokesperson Ned Price told reporters.

Blinken and Austin "pledged continued support to meet Ukraine's humanitarian, security, and economic needs," he added.

Biden last met Kuleba in Washington on February 22 -- two days before Russia began its assault.

Since then, Kuleba also met Blinken in Poland next to the border with Ukraine on March 5.

Biden is on the second and final day of a visit to Poland after he met with EU and NATO leaders in Brussels earlier in the week.

After talking to the Ukrainian ministers, Biden met with Polish President Andrzej Duda at the presidential palace.

On Friday, the US president spoke to US soldiers stationed in Poland near the Ukrainian border and aid workers helping refugees fleeing the conflict.

He praised Ukrainians for showing "backbone" against the Russian invasion and compared their resistance to the Tiananmen Square pro-democracy protests in China in 1989.

"This is Tiananmen Square squared," he said.

He also referred to Russian President Vladimir Putin as "a man who, quite frankly, I think is a war criminal".

"And I think we'll meet the legal definition of that as well," he said.

Biden said he would have liked to see the devastation caused by the conflict "first-hand".

"They won't let me, understandably I guess, cross the border," he said.

Speaking to the troops, he said: "You're in the midst of a fight between democracies and autocrats. What you're doing is consequential, really consequential."

Later on Saturday, he is due to visit a reception centre for refugees and give a major speech on the conflict.