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

"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 on Friday met Ukrainian President Volodymyr Zelensky in Paris, pledging support for the Ukrainian cause. 

The US 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 US democracy to a severe test.

Trump has also signalled his lack of interest in international organisations including NATO.

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