Russian Foreign Minister Sergei Lavrov has accused the Ukrainian Government of breaking last week's Geneva accord.
The BBC reports that Mr Lavrov said the Kiev administration - not recognised by Moscow - had not moved to disarm illegal groups, especially the ultra-nationalist-right sector.
"Extremists are calling the tune," he alleged, condemning a fatal shooting near Sloviansk, in eastern Ukraine.
Mr Lavrov says Russia is increasingly being called on to save south-eastern Ukraine - where much of the population is ethnic Russian - from chaos.
He also condemned the continuing Maidan street protests in Kiev.
But the Ukrainian Foreign Minister Andriy Deshchytsya said he was "surprised" Mr Lavrov did "not know what is being done in Ukraine regarding the Geneva agreements", Kiev-based news agency Unian reported.
The Government had been having regular consultations with the parties to the agreement in an attempt to find "ways of de-escalating the situation in the east of Ukraine", Mr Deshchytsya was reported to have said.
US presses Lavrov
Separately, US Secretary of State John Kerry has urged Russia to help implement last week's Geneva accord on resolving the Ukraine crisis.
The State Department said Mr Kerry spoke with Mr Lavrov by telephone on Monday.
The department spokeswoman Jen Psaki said he urged Russia to take concrete steps to help implement the Geneva agreement, including publicly calling on separatists to vacate seized buildings and illegal checkpoints, accept amnesty and address their grievances politically.
The European security organisation monitoring the withdrawal of pro-Russian forces from government buildings in Ukraine, says masked men at checkpoints are making it hard for its observers to gain access.
A team from the Organisation for Security and Cooperation in Europe (OSCE) is in Ukraine to oversee the implementation of the Geneva accord reached last week to ease the crisis.
A senior mediator from the group says he has met the self-appointed mayor of Sloviansk to ask whether he will comply with the agreement.