President Biden says the US and allies will respond 'decisively' if Russia invades Ukraine.
The US president spoke to Ukraine's leader about the Russian troop build-up near the country's border.
Mr Biden and President Volodymyr Zelenskyy's call came as the US and Western allies prepared for a series of diplomatic meetings to try to de-escalate a crisis that Moscow said could rupture ties with Washington.
White House press secretary Jen Psaki said in a statement: "President Biden made clear that the United States and its allies and partners will respond decisively if Russia further invades Ukraine."
Ms Psaki added that Mr Biden underscored his commitment to the principle of "nothing about you without you" – that the US will not negotiate policy that impacts Europe without its allies' input.
Mr Biden has warned his Russian counterpart Vladimir Putin that America could impose new sanctions against Russia if it takes further military action against Ukraine.
However, he said last month that US military action is not on the table.
Russia has made clear it wants a written commitment that Ukraine will never be allowed to join NATO and that the alliance's military equipment will not be positioned in former Soviet states – demands that the Biden administration has rejected.
Mr Biden told Mr Putin a diplomatic path remains open even as the Russians have moved an estimated 100,000 troops toward Ukraine and Kremlin officials have turned up the volume on their demands for new guarantees from the US and NATO.
Senior US and Russian officials are scheduled to meet on 9-10 January in Geneva to discuss the situation.
Those talks are to be followed by meetings at the NATO-Russia Council and at the Organisation for Security and Co-operation in Europe.