On Thursday, US President Joe Biden called the Ukrainian president and nine NATO allies in Eastern Europe, to confirm US support if Russia "attacks" Ukraine, as well as imposing "strict economic sanctions".

Biden spoke with Ukrainian President Volodymyr Zelensky, expressing the "deep" concerns of the United States and its European allies about Russia's "aggressive activities" towards Ukraine, and explaining that they "will respond with economic and other strong measures" in the event of military intervention.

President Biden reiterated the United States' commitment to Ukraine's sovereignty and territorial integrity, stressing that the United States and its allies are committed to the principle of "not taking decisions or discussions about Ukraine without its participation," according to a White House statement.

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Both sides called on Russia to "de-escalate tensions" and agreed that diplomacy was "the best way to make tangible progress" in resolving the conflict.

"Biden confirmed that the United States will provide, and this is what is happening, all necessary assistance so that Ukraine has the ability to respond in any way and at any time," a spokesman for Zelensky's office said in a televised statement.

Biden stressed, according to a White House statement, in his conversation with the leaders of the "Bucharest Group", which includes Bulgaria, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania, and Slovakia, the need to take a "unified, ready and resolute position to collectively defend allies." 

The US President had held a video summit with his Russian counterpart Vladimir Putin, where they discussed issues related to the internal Ukrainian crisis, while Biden warned Putin of " unprecedented US sanctions if Russia attacks Ukraine."

The Kremlin has denied any plans to invade, accusing Washington of fueling a war scare.