KIEV (Reuters) - U.S. Vice President Joe Biden told Ukrainian presidential candidates and members of parliament on Tuesday that Washington was ready to help Ukraine's economy but warned them they must fight the "cancer" of endemic corruption.

In Kiev to offer a symbolic show of U.S. support, Biden said the presidential election on May 25 would be an opportunity to lay the groundwork for a more unified and prosperous country.

Washington, he said, was ready to assist in holding it.

"You're a month away from what I would respectfully suggest ... may be the most important election in Ukrainian history," Biden said during a meeting at the parliament building.

Biden's visit to Ukraine follows the signing in Geneva last Thursday of a four-way peace deal to de-escalate tension in eastern Ukraine where pro-Russian separatists have seized towns and key facilities.

The Kiev leadership says the rebellions in the east are inspired and directed by Russia, one of the signatories of the Geneva accord, which annexed Crimea in March.

The Geneva accord is having only limited effect, however, with separatists refusing to put down their arms and pull out of key occupied points in line with the agreement.

Saying Ukraine faced humiliating threats and daunting problems, Biden said the United States was ready to assist its leaders in seizing a chance to create national unity.

"The opportunity to generate a united Ukraine, getting it right, is within your grasp. And we want to be your partner and friend," he said.

A U.S. official told reporters on Monday that Biden would discuss energy security with Ukraine's leaders, including ways to increase domestic production of natural gas. Kiev currently relies heavily on gas supplies from Moscow.

Biden told the legislators it would take time for Ukraine to achieve energy security but it was within the country's power to do so.

"Imagine where you'd be today if you were able to tell Russia: keep your gas. It would be a very different world," he said.

Biden said the United States was ready to help Ukraine stabilize and strengthen its economy, but he warned the leaders they needed to crack down on abuses within the political system.

"To be very blunt about it ... you have to fight the cancer of corruption," he said.

Billionaire confectionary oligarch Petro Poroshenko, who with former prime minister Yulia Tymoshenkois one of the front-runners in the May election, was present at Tuesday's meeting.

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