Speaking on a visit to Kyiv, U.S. Vice President Joe Biden said that the United States stands ready to help Ukraine's economy but that more must be done to fight the "cancer" of corruption.
Biden said the United States was ready to assist Ukraine, which he said faces "humiliating threats" and "very daunting problems," in seizing a chance to create national unity and energy independence.
"We want to be your partner and friend," he told Ukrainian presidential candidates and members of parliament on April 22.
He said the country's May 25 presidential election may be "the most important election in Ukrainian history."
He warned, however, that Ukraine must step up efforts to root out abuses within its political system and "fight the cancer of corruption."
Biden is on the final day of a high-profile visit to express Washington's support for the Ukrainian leadership amid the crisis with Russia.
He met with acting President Oleksandr Turchynov and is holding talks with Prime Minister Arseniy Yatsenyuk.
While in Kyiv, he is also expected to announce a package of technical assistance to Ukraine, focused on energy and economic aid.
Biden's visit comes as the United States and Russia are blaming each other for violating a peace deal to ease the country's crisis.
The Geneva agreement -- signed by Ukraine, Russia, the United States, and the European Union last week -- calls for pro-Moscow insurgents holding a string of eastern towns to disarm and leave the state buildings they have seized.
Washington says it is considering new sanctions on Russia if Moscow does not make an effort to implement the accord, while Russia has called on Washington to use its influence to pressure Kyiv to fully implement the deal, saying the pro-European protest camp in Kyiv's Independence Square must also be disbanded.
Meanwhile, Crimean Tatar leader Mustafa Dzhemilev has been barred from entering Crimea for the next five years.
A spokeswoman for the Mejlis, the Crimean Tatars' assembly, said Dzhemilev was informed he was persona non grata until 2019 as he crossed to mainland Ukraine early on April 22.
Dzhemilev has accused Crimea's new pro-Russian government of overseeing what he says is a campaign of repression against Crimean Tatars since the peninsula was annexed by Russia last month.
The Mejlis has refused to recognize Crimea's new leadership and the referendum on Crimea's secession from Ukraine.
Dzhemilev, who is also a member of Ukraine's parliament, the Verkhovna Rada, condemned the decision as "an indication of what a 'civilized' state we are dealing with."
He said he would ignore the travel ban.
With reporting by Reuters, AFP, AP, and the Crimean Unit of RFE/RL's Ukrainian Service