Russian Foreign Minister Sergei Lavrov has accused Ukraine of breaking an agreement aimed at defusing tensions in the country.
He said: “Steps are being taken – above all by those who seized power in Kiev – not only that do not fulfil, but that crudely violate the Geneva agreement.”
It came as pro-Russian separatists claimed they came under attack in the eastern city of Slavyansk.
Mr Lavrov said the deadly firefight, which reportedly left at least three dead and others wounded, showed Kiev did not want to control “extremists”.
He said: “The authorities are doing nothing, not even lifting a finger, to address the causes behind this deep internal crisis in Ukraine.”
The self-declared mayor of Slovyansk has appealed to Russia’s President Vladimir Putin to help residents by sending in troops.
This will add to fears in the West, which accuses Moscow of stoking unrest, that the violence will be used as an excuse for Russia to seize more Ukrainian territory, in a repeat of the annexation of Crimea.
Russia has tens of thousands of troops based along Ukraine’s eastern border.
The shootout threatens to derail already shaky efforts to de-escalate the continuing crisis.
The deal reached between Russia, Ukraine, the US and European Union, demanded an immediate end to violence.
The Geneva accord called for illegal armed groups to disband, and for protesters to leave occupied government buildings.
But pro-Russian separatists, who have seized offices across eastern Ukraine, have shown no signs of moving on.
Mr Lavrov pointed out the authorities in Kiev had failed to remove protesters camped out in the capital. “This is absolutely unacceptable,” he said.