Speaking after a summit of the 30 NATO leaders, and insisting he has their full support in meeting the Kremlin chief, Biden acknowledged that he faces a tough opponent in Geneva on Wednesday.

"He's bright, he's tough, and I have found that he is, as they say, a worthy adversary," he said.

"I'm not looking for conflict with Russia, but that we will respond if Russia continues its harmful activities."

Where Washington and Moscow do not agree, he "will make it clear what the red lines are," he said.

One of these was "the sovereignty and territorial integrity of Ukraine".

Biden vowed that, faced with "aggressive action" from Russia, "we're going to put Ukraine in a position to maintain their physical security".

But, despite pressure from Ukraine's President Volodymyr Zelensky to use the moment to push for Kiev's admission to NATO, Biden warned that remained conditional on Ukraine adopting democratic reforms and fighting corruption.