US President Joe Biden warned Saturday that Vladimir Putin would "not stop" at Ukraine as he pledged Washington's support for Kyiv in its battle against the Russian invasion.

"Putin is not going to stop at Ukraine," Biden said alongside French President Emmanuel Macron after talks in Paris.

"All of Europe will be threatened, we are not going to let that happen," Biden told journalists during his state visit to France.

"The United States is standing strong with Ukraine. We will not, I say it again, walk away," he added.

Macron told Biden in front of reporters: "I thank you, Mr. President, for being the president of the world's number one power but doing it with the loyalty of a partner who likes and respects the Europeans."

Both Biden and Macron met Ukrainian President Volodymyr Zelenskyy in Paris on Friday, pledging support for the Ukrainian cause.

The U.S. president, 81, has been in France since Wednesday and took part in this week's commemorations marking the 80th anniversary of the D-Day landings that changed the course of World War II.

On Friday, Biden warned of the need to preserve American democracy and drew parallels between World War II and Russia's invasion of Ukraine.

Biden is set to face his Republican rival and predecessor Donald Trump later this year in presidential elections that commentators predict will subject U.S. democracy to a severe test.

Trump has also signaled his lack of interest in international organizations including NATO.

"We have shown the world again the power of allies and what we can achieve when we stand together," said Biden.