US Vice President Joe Biden has called on Ukraine's new leaders to fight the "cancer of corruption" in the country.
Mr Biden was speaking on the second day of his visit to Kiev, where he stressed that the forthcoming presidential election was the most important in the country's history.
He also pledged that the Obama administration would do what it could to help create a united Ukraine.
Tensions remain high in eastern Ukraine after at least three died in a day of shooting during what was supposed to be an Easter truce.
In an exclusive interview with NBC News, the Ukrainian Prime Minister warns of the dangers of unchecked Russian expansion and aggression.
Russia, the US and the EU say that all sides have agreed to steps to "de-escalate" the crisis in Ukraine at talks in Geneva.