Talks are due to take place on both sides of the Atlantic to try to find a solution to the crisis with Russia.
French President Emmanuel Macron is travelling to Moscow and the German chancellor is in Washington to meet President Biden.
Vladimir Putin denies planning an invasion - despite massing troops on Ukraine's border.
US national security adviser Jake Sullivan says Russian action could be imminent.
"It could happen as soon as tomorrow or it could take some weeks yet.
"He has put himself in a position with military deployments, to be able to act aggressively against Ukraine at any time now."
