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KIEV, Ukraine (CNN) — As the crisis in Ukraine shows no signs of easing, U.S. Vice President Joe Biden promised support for Ukraine and stressed that the United States won’t recognize Russia’s annexation of Crimea.

“Ukraine is and must remain one country,” he said in Kiev on Tuesday at a news conference with Prime Minister Arseniy Yatsenyuk. Biden also met with acting President Oleksandr Turchynov and lawmakers during his trip to Ukraine.

“No nation has the right to simply grab land from another nation,” Biden said. “We will never recognize Russia’s illegal occupation of Crimea.”

Biden called on Russia to “stop supporting men hiding behind masks and unmarked uniforms sowing unrest in eastern Ukraine.” He warned of additional sanctions if such “provocative behavior” does not end.

Ukrainian and U.S. officials say they think Russian special forces are in the region and are behind efforts to seize government buildings and generally promote unrest.

The U.S. vice president promised financial support, assistance in reducing Ukraine’s dependence on Russian energy sources and nonlethal aid for security forces.

“You will not walk this road alone. We will walk it with you,” Biden told Yatsenyuk.

As he spoke, the White House announced a new $50 million package of assistance to help Ukraine pursue political and economic reform and strengthen its partnership with the United States.

Biden also said he expects an International Monetary Fund package for Ukraine to be finalized imminently.

The backing is likely to sit well with Ukrainian leaders struggling to keep their country afloat amid dire financial problems, the ongoing showdown with Russia over its annexation of Crimea and alleged interference in Ukraine’s pro-Russian east.

Tensions remain high as pro-Russian militants show no sign of leaving occupied government buildings in eastern Ukraine despite an international deal agreed to in Geneva, Switzerland, last week.

‘Humiliating threats’

In his meeting with lawmakers and the acting President, Biden again stressed that the United States wants to stand with the people of Ukraine.

Ukraine faces very daunting problems and “some might say humiliating threats,” he said in an apparent reference to Russian interference, but success is within its grasp.

“We want to be your partner and friend, we want to assist,” Biden said. “We are not suggesting we have all the answers. We are suggesting we stand with you in every endeavor you undertake.”

Biden also issued a warning to the leaders in Ukraine, urging them to “fight the cancer of corruption that’s endemic” in the country and reform the judicial system.

It won’t be easy, he said, but sometimes “a crisis spawns the commitment, and the desire, the willingness to make some of these bold decisions.”

About $11.4 million of the new $50 million aid package is destined to support Ukraine’s planned May 25 presidential vote, which Biden suggested could be “the most important election in Ukrainian history.”

And as a step toward reducing Ukraine’s reliance on Russian gas, U.S. experts are in Kiev “to help Ukraine secure reverse flows of natural gas from its European neighbors,” the White House said.

The promise of further economic help comes on top of a $1 billion loan guarantee already signed by the United States.

The White House also announced Tuesday another $8 million in nonlethal military assistance for Ukraine’s military and border service. It is already providing nearly $10 million in nonlethal aid.

Turchynov: Ukrainians want unity

Turchynov, meanwhile, had strong words for Moscow, saying that “Ukrainians are ready to protect their country from military aggression of the Russian Federation.”

A majority of Ukrainian citizens “are for a united, democratic Ukraine with greater powers given to the regions,” he said, referring to constitutional reforms promised in a bid to calm fears in eastern Ukraine.

Kiev and Western powers have urged Moscow to use its influence to persuade protesters in the east to lay down their arms, leave the buildings they’re occupying and accept amnesty in line with last week’s deal.

But Turchynov said Tuesday that “unfortunately, the Russian Federation and its terrorist special forces present in Ukraine are demonstratively not going to implement the Geneva agreement.”

He cited the seizure of a police station Monday in the eastern city of Kramatorsk by militants who also took the police chief hostage, “thereby negating the agreements reached in Geneva.”

Moscow has said that it holds no sway over pro-Russian protesters and militants. But Kiev and the United States dispute this.

In a phone call Monday with his Russian counterpart, U.S. Secretary of State John Kerry urged Russia to take “concrete steps” to help implement the Geneva deal and de-escalate the situation.

Russian Foreign Minister Sergey Lavrov countered by saying that Ukrainian officials were not implementing the April 17 agreement.

“On the contrary, not a single step has been taken by those who have seized power in Kiev to eliminate the reasons of this deep crisis inside Ukraine,” he said.

Meanwhile, a statement from the prosecutor general’s office in Kiev criticized law enforcement agencies in a number of cities in eastern Ukraine for not resisting pro-Russian protesters as they seized administrative buildings — even in cases where they outnumbered the militants.

The statement says self-defense groups in these places were established in reaction to the failings of the security forces. “These defense groups have the right to bear arms and promise to guarantee public order in their respective regions,” it said.

Ukraine: Photos show undercover Russian forces

Ukrainian officials said Monday that a dossier of photos obtained by CNN show Russian “sabotage-reconnaissance groups” acting in Ukrainian towns.

The officials said the images prove organized Russian activity in the region.

CNN cannot independently confirm the photographs, some of which were first published in The New York Times.

The dossier shows what Ukrainian officials say are images of well-equipped gunmen in eastern Ukraine, who look similar to photographs of Russian forces taken in Crimea, Russia and during Russia’s 2008 invasion of Georgia.

Last week, Ukrainian security officials told CNN they had arrested a Russian military officer and a woman who Ukrainian officials said is a Russian intelligence agent.

Moscow has disavowed involvement in the takeover of government buildings in eastern Ukraine or other acts by often masked pro-Russian gunmen.

But the photos, accepted as genuine by the Obama administration, appeared to lend credence to allegations by Ukrainian officials that Russian forces have been dispatched in eastern Ukraine to provoke a military confrontation.

If genuine, the photos also back up Western leaders who have alleged Russia’s involvement.

‘Men in green’

The question of whether Russia is involved in the unrest roiling eastern Ukraine is crucial as European observers try to enforce the agreement reached last week in Geneva by organizing the withdrawal of forces from government buildings and other facilities.

CNN has heard Russian accents among the “men in green,” as they are known, well-armed and uniformed groups that have appeared in towns like Slaviansk and Kramatorsk. One told CNN he had come up from Crimea. But CNN has not seen any evidence that these men are operating under orders from the Kremlin.

Lavrov scoffed at the accusations Monday, saying that Kiev and its patrons, the United States and the European Union, are trying to blame his country for everything.

Meanwhile, the stalemate continues.

The government of Turchynov, the acting President, has talked tough but done little to curb pro-Russian activities in the east, possibly afraid that a crackdown could send Russian forces across the border. At the same time, the occupation of buildings continues in about a dozen towns and cities across eastern Ukraine.

In three towns, pro-Russian protesters and militants have made it clear to CNN they have no intention of moving until the “illegal” government in Kiev also moves out of official buildings.

Moscow also says the government in Kiev, installed after ousted President Viktor Yanukovych fled Ukraine for Russia in February, is illegitimate and backed by far-right extremists.

Yanukovych’s ouster followed months of street protests prompted by his decision last November to drop a planned EU trade agreement in favor of closer ties with Moscow.

Russia annexed Ukraine’s autonomous Crimea region last month, following a controversial referendum in which most voters were in favor of joining the Russian Federation.

CNN’s Gul Tuysuz reported from Kiev and Laura Smith-Spark wrote from London. CNN’s Arwa Damon, Joe Sterling, Michael Pearson, Ed Payne and Becky Brittain, and journalist Victoria Butenko contributed to this report.

By Laura Smith-Spark and Gul Tuysuz