U.S. President Barack Obama is due to hold talks with European Council President Donald Tusk in Washington.
The White House has said the two are to discuss the conflict in Ukraine on March 9, among other topics.
Violence in eastern Ukraine has dropped sharply in the past two weeks after a peace accord brokered in Minsk last month by France and Germany.
Ahead of his meeting with Obama, Tusk said in an interview with The New York Times on March 8 that Europe was not yet prepared to step up sanctions against Moscow despite provocations by what he called the Russian "aggressor."
EU foreign ministers decided at talks in Riga on March 7 to give the fragile cease-fire a chance before debating whether to impose more sanctions or even to extend existing ones.
Tusk said the cease-fire in eastern Ukraine had clearly not put an end to the fighting, but at least led to a "situation that is better than before."
However, he warned that a new surge in fighting by pro-Russian rebels would result in Europe beefing up sanctions.
Tusk, a former prime minister of staunchly pro-U.S. Poland, took over as president of the European Council in September -- replacing Herman Van Rompuy and becoming the first East European to hold one of the bloc’s most senior positions.
Tusk told the U.S. newspaper that trans-Atlantic relations were "absolutely the backbone not only of our two continents," but also "the only guarantee that our values like democracy, free markets, free movement, and human rights are based on real power."
Tusk said in his interview that the conflict in eastern Ukraine and Russia’s annexation of Crimea in March 2014 were "just one of many" crises that now threaten Europe's security and require a concerted response by the European Union and the United States.
He said this was "the first chapter in a new and very dynamic history of new fears and new threats," citing among other security challenges the growing chaos in Libya and the threat of violence by Islamic militants.
