The United States has offered Ukraine a new $50 million aid package to help with economic and political reform, the White House said on Tuesday during a visit to Kiev by Vice President Joe Biden.

It also offered an additional $8 million in non-lethal military aid, including radios and vehicles, according to a statement issued by Biden's staff in Kiev.

Small in terms of Ukraine's needs and in relation to the $1 billion loan guarantee already signed with Washington, the package, along with Biden's visit, was a clear show of support for the new authorities following the overthrow of the Kremlin-backed president and continuing 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," it said.

Saying Washington was ready to provide further assistance after a presidential election on May 25, it said $11.4 million of the package would support the integrity of that vote.

Biden told Ukrainian members of parliament in Kiev, including presidential candidates, that the United States was ready to help Ukraine's economy but warned that they must fight the "cancer" of endemic corruption.

He said the presidential election on May 25 would be the most important in the country's history and Washington was ready to help in holding it. 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.

He said there was no reason Ukraine could not achieve energy security, but it would take time. He asked the legislators to imagine the stronger position Kiev would be in now if it was not dependent on Russia for gas supplies.

Biden's trip comes days after the U.S., Russia, Ukraine and Europe signed an agreement in Geneva calling for Moscow to use its influence to get pro-Russian forces to leave the numerous government buildings they now occupy in cites throughout eastern Ukraine. The U.S. asserted on Monday that publicly available photographs from Twitter and other media show that some of the troops in eastern Ukraine are Russian special forces, and the U.S. said the photos support its case that Moscow is using its military to stir unrest in Ukraine.

There was no way to immediately verify the photographs, which were either taken from the Internet or given to the Organization for Security and Cooperation in Europe last week by Ukraine diplomats.

In Moscow, Foreign Minister Sergey Lavrov rejected charges that Moscow was behind the troubles in eastern Ukraine and failing to live up to the Geneva agreement.

"Before putting forth ultimatums to us, demanding fulfillment of something within two-three days or otherwise be threatened with sanctions, we would urgently call on our American partners to fully recognize responsibility for those whom they brought to power and whom they are trying to shield, closing their eyes to the outrages created by this regime and by the fighters on whom this regime leans," Lavrov told a news conference.

Words and actions by Ukrainian leaders are "absolutely unacceptable," he declared.

The U.S. has warned that it will quickly order new economic sanctions on Russian officials and entities if Moscow doesn't follow through on the provisions in last week's accord. Gregory Pyatt, the U.S. ambassador to Ukraine, said it was still too early to tell whether the deal would succeed, but he added, "The ball is really in Moscow's court in terms of whether they're going to take this diplomatic off-ramp."

"There needs to be concrete results," Pyatt told reporters in Kiev. He said the U.S. would make a decision on whether the agreement is working in "days, not weeks."

While last week's agreement offered a glimmer of hope that the crisis in Ukraine could be resolved peacefully, the accord appears to be fragile at best. The armed pro-Russia groups have refused to leave their occupying positions in eastern Ukraine until the country's acting government resigns. And there was a burst of violence Sunday, with three people killed during a shootout at a checkpoint that was manned by pro-Russian troops.

Ukrainian and Russian officials each blamed the other for instigating the attack. The White House said it was still trying to determine who was responsible and had no independent verification of what transpired.

"Overall, we are concerned about the situation there, and we urge paramilitary groups throughout the eastern and southern parts of Ukraine to lay down their weapons and depart the buildings that they have occupied, as was called for in the accord signed in Geneva last week," White House spokesman Jay Carney said.

Even as officials sorted through this latest disturbance, the State Department sought to build a public case against Russia for the wider unrest. The photo images released Monday show militants brandishing Russian weapons and wearing uniforms similar to those worn by Russian forces. The militants look similar to the forces that moved into Crimea in March, ahead of a referendum there that resulted in the peninsula being annexed by Russia.

Secretary of State John Kerry and Lavrov spoke by telephone Monday but appeared to break little new ground. Russia's foreign ministry said Lavrov told Kerry that the Ukrainian government was unable and unwilling to stop what the Russians call extremists in eastern Ukraine.