U.S. Vice President Joe Biden has vowed support for Ukraine, which he said "is in a struggle for its very future."
Biden said the road ahead is difficult, but that Ukrainians "will not walk this road alone."
He was speaking on April 22 after talks in Kyiv with Prime Minister Arseniy Yatsenyuk.
Biden said "there are some who are trying to pull Ukraine apart," but added that Ukraine is and must remain one united country.
He also said it is time for Russia to fulfill its commitments under a peace agreement signed last week in Geneva.
Biden said it is time for Russia "to stop talking and start acting" to reduce tension in Ukraine. "We will not allow this to become an open-ended process," he said.
He warned that Russia faces "more cost and greater isolation" if it fails to defuse tensions.
Washington and Moscow are blaming each other for violating the Geneva agreement, signed by Ukraine, Russia, the United States, and the European Union last week.
The agreement calls for pro-Moscow insurgents holding a string of eastern towns to disarm and leave the state buildings they have seized.
Biden also announced that the United States will provide an additional $50 million to help Ukraine's government with political and economic reforms.
Separately, Washington has also offered $8 million in nonlethal assistance for the Ukrainian armed forces, including bomb-disposal equipment, communications gear, and vehicles.
Biden earlier called on Ukraine's new government to fight the "cancer" of what he called endemic corruption in the country.
For his part, Yatsenyuk said that Russian special forces were operating like "gangsters" in eastern Ukraine to undermine the May 25 presidential election, and he called on Moscow to pull them out.
WATCH: Ukraine's acting Prime Minister Arseniy Yatsenyuk
He also called on Moscow to pull its army out of Crimea and "turn this shameful page in which Ukrainian territory has been seized by Russian troops."
Biden is the most high-level U.S. official to visit Ukraine since the crisis erupted.
Earlier on April 22, the final day of his visit, Biden held talks with acting President Oleksandr Turchynov.
Biden also met leaders from various political parties, including three candidates in the May 25 presidential election.