* Biden visit to Kiev offers support, caution to Kiev
* Demands Russia cooperate, threatens further sanctions
* U.S. aid for "historic" Ukrainian election on May 25
* Biden tells Kiev leaders to fight "cancer of corruption"
By Jeff Mason and Pavel Polityuk
KIEV, April 22 (Reuters) - In a show of U.S. support for Ukraine's embattled government, Vice President Joe Biden delivered an aid package on Tuesday and demanded Russia back off but also warned Kiev it must tackle the "cancer of corruption".
Demanding Moscow "stop talking and start acting" to disarm pro-Russian separatists who Ukraine says aim to undermine the May 25 election, Biden told presidential candidates it may be "the most important election in Ukrainian history".
Part of the $50-million aid package was earmarked to support the "integrity" of the vote. But after 23 years of post-Soviet independence from Moscow that have seen endemic graft sap the economy and public faith in the state, Biden told lawmakers: "To be very blunt ... you have to fight the cancer of corruption."
Prime Minister Arseny Yatseniuk, who took office after the overthrow of Ukraine's Kremlin-backed president two months ago, told him that this would be a priority of the new administration.
Repeating a threat of heavier economic sanctions after those impose following the annexation of Crimea last month, Biden renewed U.S. calls for Russia to pull back troops from Ukraine's eastern border and said "time is short" for Moscow for show it is upholding its commitments under a four-way agreement signed at Geneva on Thursday that is intended to defuse the crisis.
Ukraine and its U.S. and European Union allies, which signed the accord with Russia, complain Moscow is failing to persuade separatists in the east to disarm and vacate occupied public buildings. Russia denies it is orchestrating the militants, who say they want the chance to join Crimea in being annexed by Moscow.
"No nation should threaten its neighbours by amassing troops along the border. We call on Russia to pull these forces," Biden told a joint news conference after meeting Yatseniuk.
"We have been clear that more provocative behaviour by Russia will lead to more costs and to greater isolation," the vice president said. "We've heard a lot from Russian officials in the past few days. But now it's time for Russia to stop talking and start acting."
Biden added: "We will not allow this to become an open ended process. Time is short in which to make progress."
Yatseniuk said: "Everything that is now happening in the east and which Russia is supporting is aimed at wrecking the presidential election.
"We demand that our Russian neighbours immediately recall their special forces, which are in the east, recall the army from Crimea and turn this shameful page in which Ukrainian territory has been seized by Russian troops."
Of the $50 million in new aid, the White House said in a statement, $11.4 million was earmarked for helping with the election to choose a successor to President Viktor Yanukovich.
It also offered an additional $8 million in non-lethal military aid, including radios and vehicles.
Small in terms of Ukraine's needs and in relation to the $1 billion loan guarantee already signed with Washington, the aid package, along with Biden's visit, was a clear show of support for the new authorities in the confrontation with Russia.
"The United States is committed to ensuring that Ukrainians alone are able to determine their country's future without intimidation or coercion from outside forces," the statement said. U.S. experts would also work on reducing Ukraine's dependence on Russian gas and in fighting corruption.
In a meeting with leading members of parliament, including several candidates for the presidency, Biden spoke of the "heroism" of Ukrainians and of the "humiliating threats" they face in trying to create a united nation: "Getting it right is within your grasp," he said. "And we want to be your partner, your friend in the project. And we're ready to assist."
In reference to disenchantment with the lost opportunities of independence and of the Orange Revolution a decade ago that promised a new start, Biden said: "You have an opportunity, a chance to bring about an era of reform and democratic renewal that you all hoped for two, five, 10, 15 years ago, to lay the groundwork for an even more united and more prosperous Ukraine."
Touching on the country's dependence on Russia for gas, which has limited Kiev's options in negotiations with Moscow, the United States is offering advice on diversifying Ukraine's energy supply. Biden said it would take time, but added:
"Imagine where you'd be today if you were able to tell Russia: keep your gas. It would be a very different world."
As well as the prime minister, Biden met parliamentary speaker Oleksandr Turchinov, who is acting president, and a group of lawmakers who included presidential candidates Petro Poroshenko, Oleh Tyagnibok and Serhiy Tigipko.
Former prime minister Yulia Tymoshenko, who is not a member of parliament but is leading presidential candidate, was invited but was unable to attend, a U.S. official said. (Additional reporting by Alastair Macdonald, Richard Balmforth and Natalia Zinets in Kiev; Writing by Alastair Macdonald; editing by David Stamp)