Russian Foreign Minister Sergey Lavrov accused Ukraine's government Monday of violating a peace agreement reached last week after an Easter shooting left three pro-Russian separatists dead at a checkpoint near the flashpoint city of Slavyansk.
The violence and traded accusations over who was behind it will be a sharp test for the new, fragile diplomatic achievement just as U.S. Vice President Joe Biden arrived in Ukraine.
The pro-Russians manning the checkpoint at Bylbasivka told CBS News' Holly Williams that members of a far right-wing, ultra-nationalist Ukrainian group attacked them in the early morning hours with sniper fire and stun grenades.
Bullet casings found at the scene seem inconsistent with the gun battle described to CBS News, however, and the far-right group has denied any involvement whatsoever, instead accusing Russian special forces of staging the attack.
The Ukrainian Security Service also said the attack was staged by provocateurs from outside the country.
Evidence allegedly showing the Ukrainian nationalist group was behind the attack was presented on Russian television, but some of the items -- particularly a pristine business card said to have been left behind by the attackers -- were met with widespread ridicule in Ukraine.
Lavrov's remarks made it clear, however, that, at least as far as Moscow is concerned, it was nationalists behind the violation of the truce deal.
"Steps are being taken -- above all by those who seized power in Kiev -- not only that do not fulfill, but that crudely violate the Geneva agreement," he said, according to the Reuters news agency.
"The authorities are doing nothing, not even lifting a finger, to address the causes behind this deep internal crisis in Ukraine," added Russia's top diplomat.
With the violence testing the recently forged truce agreement, U.S. Vice President Joe Biden arrived in Ukraine Monday for talks with country's government aimed at deescalating tensions.
The White House said President Barack Obama and Biden agreed he should make the two-day visit to the capital city to send a high-level signal of support for reform efforts being pushed the new government.
Biden has scheduled a series of meetings Tuesday, including with Arseniy Yatsenyuk and Oleksandr Turchynov, the acting Ukrainian prime minister and president. He also is scheduled to meet with legislators from across the country and democracy activists before returning to Washington Tuesday night.
A senior administration official told reporters onboard Air Force Two en route to Kiev that Biden plans 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 plans to discuss preparations for next month's 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 concentrated in the region.
The agreement reached Friday in Geneva by Russia, the U.S., Ukraine and the European Union calls for armed groups to leave occupied buildings and disarm in the east and in Kiev. Thus far, neither pro-Russian insurgents in the east nor Ukrainian nationalist demonstrators in Kiev have budged.
In the center of Slavyansk, armed, masked pro-Russian militants remain in control of the streets. They also occupy public buildings in about nine other cities, CBS News' Williams reports.
Slavyansk's self-appointed, separatist mayor said Sunday that he wants Russian troops to help defend the town. That plays into the fears of Ukraine's leaders - who accuse Moscow of stirring up this insurgency.
Both Ukraine's government and the U.S. worry that eastern Ukraine could see a re-run of what happened in Crimea just a few weeks ago, when separatist demonstrations were used as a pretext for Russia to send in thousands of troops and claim the region for itself.