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 May 25, a vote he called perhaps “the most important in Ukraine’s history.”

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.

State Security officials in Kiev say groups such as the People’s Army of the East are being directed by Moscow, with support on the ground from Russian military special forces­ and intelligence agents.

Valentyn Nalyvaichenko, director of Security Service of Ukraine, speaking Tuesday on a YouTube link with the Atlantic Council, said: “There are up to seven or eight groups of people in Lugansk. Mostly they are veterans of Afghanistan, some of them former Ukrainian special forces­ from the Soviet times. Some of them are young men from radical organizations.”

Across eastern Ukraine, Nalyvaichenko said, there are 30 officers of Russia’s foreign intelligence agency, known as GRU, “and hundreds and hundreds of those we call agents and the network of this subversive and hostile activity.”

“They are well-armed. They prepared for years to do what they are doing now,” he said. “In the last month and a half, we as the Security Service managed to arrest and detain 21 of them. They are agents. There are three officers of GRU, whom we keep in Kiev behind bars.”

Nalyvaichenko said that U.S. intelligence is being shared with the agency and that the United States has started “real, real cooperation in the security effort.”

Despite a U.S.-brokered agreement last week between Ukraine and Russia to de-escalate tensions, little has been done to corral the pro-Russia groups in the eastern region.

Russian flags fly over a few dozen government buildings, city halls and police stations in eastern Ukraine. Some of the buildings remain heavily fortified, taken over by the activists. Other buildings are open for business, and residents renew driver’s licenses or register births, as only a small group of unarmed self-defense militia members loiter outside.

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 — including providing Ukraine’s various regions authority over budgets and over cultural decisions about language — and promote the nation’s integration with Europe. Russian President Vladimir Putin has criticized such a move.

Russia “should not behave as gangsters in this modern century,” Yatsenyuk said, speaking with Biden by his side.

Biden concluded his visit by announcing that the United States would provide an additional $50 million in assistance to Ukraine, including $11.4 million to help conduct the election, as well as expertise to assist Ukraine in reducing its reliance on Russian energy supplies.

Biden also promised a modest increase of $8 million in non­lethal security aid to the Ukrainian armed ­forces and border guards, who have been confronting pro-Russia groups in the country’s eastern regions. The aid comes on top of a $10 million package announced earlier.

In Washington, Rear Adm. John Kirby, the Pentagon press secretary, told reporters that the troop deployment would begin Wednesday, when about 150 paratroopers based in Vicenza, Italy, arrive in Poland. Three other paratrooper companies from the 173rd Infantry Brigade Combat Team will arrive in Lithuania, Latvia and Estonia no later than Monday, Kirby added.

“Since Russia’s aggression in Ukraine, we have been constantly looking at ways to reassure allies and partners,” Kirby said. “If there’s a message to Moscow, it is the same exact message, that we take our obligations very, very seriously on the continent of Europe.”

He said that the exercises were expected to last about a month but that the paratroopers would be replaced by other U.S. troops on a rotational basis. “What we’re after here is a persistent presence,” he added.

Although Poland and the Baltic states are members of NATO, Kirby said the troop deployment was not part of a NATO mission. Leaders of those countries have been pressing the Obama administration to enhance the U.S. military presence inside their borders as an added deterrence to Russia.

“It’s more than symbology,” Kirby said. “The kind of work that we’re going to be doing is real infantry training. And that’s not insignificant.”

Kirby said the troop deployments could expand to other allied countries in the region, but he did not elaborate. In addition, he acknowledged that NATO has long been planning to hold military exercises in Ukraine this summer.

Booth reported from Lugansk. Karen DeYoung and Craig Whitlock in Washington contributed to this report.