Russia has "days, not weeks" to abide by an international accord aimed at stemming the crisis in Ukraine, the top U.S. diplomat in Kiev warned Monday as Vice President Joe Biden launched a high-profile show of support for the pro-Western Ukrainian government. Russia in turn accused authorities in Kiev of flagrantly violating the pact and declared their actions would not stand.

Biden, the highest-ranking American official to visit Ukraine during its conflict with Russia, planned to meet with government officials in the capital of Kiev Tuesday. The vice president also planned to announce new technical support to help the fledgling government with energy and economic reforms.

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.

Biden planned to meet Tuesday with government leaders who took over after pro-Russia Ukrainian President Viktor Yanukovych was ousted in February following months of protests. He will speak with Arseniy Yatsenyuk and Oleksandr Turchynov, the acting Ukrainian prime minister and president. The vice president is also scheduled to meet with legislators from across the country and democracy activists before returning to Washington Tuesday night.

He held a series of meetings Monday with U.S. Embassy officials, members of Congress also in Kiev for an update on the crisis and the Organization for Security and Co-operation in Europe's chief monitor in Ukraine.

A senior administration official told reporters onboard Air Force Two en route to Kiev that Biden planned to announce new technical support to the Ukrainian government to implement energy and economic reforms. The official, speaking on a condition of anonymity to allow Biden to publicly announce any agreements, said the vice president also will follow up on recent U.S. commitments of non-lethal security assistance and discuss what more Washington can offer to help.

Biden also planned to discuss preparations for next month's Ukrainian presidential election and the latest developments in eastern Ukraine, where insurgents are accusing leaders in Kiev of aiming to suppress the country's Russian speakers in the region.

The Obama administration official told reporters that the assistance Biden will announce includes technical expertise to increase production and boost energy efficiency to reduce reliance on oil imports from Russia. The economic help includes advice to make sure international funding is allocated effectively and that all parts of the country are benefiting.

