KIEV, Ukraine - Vice President Biden pledged additional American aid Tuesday to help the government here, as the Pentagon announced that it would respond to Russia's involvement in Ukraine by sending about 600 U.S. troops to conduct exercises and training in Poland and the three Baltic states.

The announcements indicated robust U.S. support for the tenuous new Ukrainian government, even as Biden warned the political class here that it must confront "the cancer of corruption that is endemic in your system right now."

Meeting with Ukrainian lawmakers, Biden expressed sympathy for the challenges they confront, squeezed between the expectations of protesters in the capital and by pro-Russia activists in eastern Ukraine, where armed groups have occupied public buildings and are demanding a referendum to consider seceding into autonomous states.

"You face very daunting problems and, some might say, humiliating threats," Biden said. "But the opportunity to generate a united Ukraine, getting it right, is within your grasp. And we want to be your partner and friend in the project."

Biden said the U.S. aid would help Ukraine defy Russian economic pressure and stage a presidential election on May 25, a vote he called perhaps "the most important in Ukraine's history."

Biden, the highest-ranking U.S. official to visit Ukraine since Russia's move into Crimea two months ago, demanded Tuesday that Russia push the groups under its sway to vacate the government buildings they have occupied and to send representatives to work with international monitors in the volatile region.

"It's time for Russia to stop talking and start acting," Biden said. "We need to see these concrete steps, and we need to see them without delay."

Biden met for more than an hour in Kiev with Prime Minister Arseniy Yatsenyuk, an administration favorite. Afterward, Yatsenyuk said the next president must carry out constitutional reforms and promote the nation's integration with Europe.

The goal of a united Ukraine seemed distant Tuesday in the impoverished industrial towns of the country's east, where an uprising against Kiev, inspired by Russia's annexation of Crimea last month, threatens to not only derail the election but also splinter the state.

In Lugansk, a separatist cadre opposed to the central government in Kiev was fortifying its position in the Ukraine State Security building that it had stormed more than two weeks ago.

The facility contained a large armory, including explosives. The men, heavily armed with Kalashnikov rifles and grenades, were holed up in dank rooms protected by sandbags, chain-smoking and listening to pro-Russia pop music. They called themselves the People's Army of the East.

"As long as we enjoy the support of our citizens, we won't leave until our demands are met," said Sergiy Gerachov, one of the commanders, dressed in fatigues and sprawled on a couch with a pair of pistols holstered around his waist and a Soviet war medal pinned to his chest.