Russian Foreign Minister Sergei Lavrov has accused the Kiev authorities of breaking last week's Geneva accord on resolving the Ukraine crisis.
He said the Kiev government - 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.
He also condemned the continuing Maidan street protests in Kiev.
He said it was "absolutely unacceptable" that the Ukrainian authorities had failed to end what he called the illegal protests in the capital.
However, 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.
Early on Sunday at least three people were killed in a shooting at a checkpoint manned by pro-Russian separatists near Sloviansk.
The circumstances remain unclear. The local separatists said the attack was carried out by Right Sector militants. Kiev called it a "provocation" staged by Russian special forces.
Mr Lavrov said the incident proved Kiev did not want to control "extremists".
He said that the most important demand of the Geneva deal was to "prevent any violence", and it was not being implemented.
"Steps are being taken - above all by those who seized power in Kiev - which crudely violate the accords reached in Geneva," Mr Lavrov told a news conference in Moscow.
The 17 April Geneva accord was agreed at talks between Russia, Ukraine, the EU and US. It demanded an immediate end to violence in eastern Ukraine and called on illegal armed groups to surrender their weapons and leave official buildings.
Pro-Russian militants are still holding official buildings in at least nine towns and cities in the Donetsk region of eastern Ukraine.
The interim authorities in Kiev said they had suspended operations against pro-Russian militants over Easter, and appealed for national unity.
They promised to meet some of the demands of pro-Russian protesters, which include the decentralisation of power and guarantees for the status of the Russian language.
The job for Western diplomats trying to bridge the divide is getting harder, says the BBC's Tom Esslemont in Moscow.
The Organisation for Security and Cooperation in Europe (OSCE) has deployed around 100 monitors across 10 cities in Ukraine to explain the details of the Geneva accord to each side.
Spokesman Michael Bociurkiw said they were having a "mixed experience dealing with checkpoints and so forth and there is a varying reaction to teams".
He said they were facing a "hardened attitude" in places such as Donetsk and Slaviansk, but other, smaller, areas are "more accommodating".
Meanwhile, US has warned the next few days will be pivotal and has threatened more sanctions against Russia if it fails to abide by the Geneva accord.
US Vice-President Joe Biden arrived in Kiev on Monday for two days of meetings with the country's leaders, in a show of support for the Ukrainian government.
Mr Biden will announce technical support to the Kiev government, including economic and energy-related assistance.
Mr Lavrov stressed any attempt to isolate Russia from the rest of the world was doomed to fail.
"We are a big and independent power that knows what it wants," he said, adding that "the overwhelming majority of states in the world do not want Russia's isolation."
Ukraine has been in crisis since last November, when Kiev was gripped by protests against President Viktor Yanukovych over his rejection of an economic pact with the EU. He was toppled in February and fled to Russia.
Russia then annexed Crimea following a regional referendum that approved joining the Russian federation. The annexation provoked international outrage.
Russian President Vladimir Putin on Monday submitted a bill to parliament to establish a gambling zone in Crimea.
He also signed a decree to rehabilitate Crimea's Muslim Tatars and other ethnic minorities who suffered during the rule of Soviet dictator Josef Stalin.
The 300,000-strong Tatar community - which makes up 15% of Crimea's population - opposed the peninsula's incorporation into Russia last month.