US President Joe Biden on Friday compared Russia's invasion of Ukraine to China's crushing of protests in Tiananmen Square in 1989, speaking during a visit to Poland near the border with Ukraine.
Biden also referred to Russian President Vladimir Putin as "a man who, quite frankly, I think is a war criminal", adding: "And I think we'll meet the legal definition of that as well."
Biden spoke at meetings with US soldiers stationed in Poland close to the border and with aid workers helping to deal with the massive refugee crisis caused by the conflict in Ukraine.
The US president 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.
Biden praised Ukrainians for showing "backbone" in their resistance against Russia, giving the example of "a 30-year-old woman standing there in front of a tank with a rifle".
"I mean, talk about what happened to Tiananmen Square. This is Tiananmen Square squared," 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."
Biden landed earlier on Friday in the city of Rzeszow in southeastern Poland -- around 80 kilometres (50 miles) in a straight line from the border with Ukraine.
Biden will then travel on to Warsaw to meet Polish leaders and visit a reception centre for refugees.
He is due to wrap up the visit on Saturday with a major speech.
Poland is an EU and NATO member that has taken in millions of Ukrainians, offered financial support to Ukraine and sent arms.
White House National Security Adviser Jake Sullivan said on Friday that there were around 10,500 US soldiers currently in Poland and reiterated Biden's promise to defend "every inch" of NATO territory.