Ukraine's acting president has said his government will launch a "full-scale anti-terrorist operation" involving the country's army against pro-Moscow separatists, claiming Russia was waging a war against Ukraine.
Oleksandr Turchynov said on Sunday in a televised address to the nation that he offered not to prosecute any armed rebels who gave up their weapons by Monday 0600 GMT, Reuters news agency reported.
Russia's Foreign Ministry said later on Sunday that the announcement was a "criminal order", and that it was "now the West's responsibility to prevent civil war in Ukraine".
"We decisively condemn attempts to use brute force against protesters and activists," the statement of the ministry said. "We are particularly indignant about the criminal order to use the army to put down protest."
Ukraine's ousted president, Viktor Yanukovich, later accused the CIA of being behind Kiev's decision to deploy troops, claiming CIA director John Brennan had met Ukraine's new leadership and "in fact sanctioned the use of weapons and provoked bloodshed".
There was no independent confirmation that Brennan was in Ukraine of in any way involved in the decision.
Earlier on Sunday Ukraine's security forces announced their efforts to end the grip of separatists on Slovyansk.
"An anti-terrorist operation has begun in Slovyansk. It is being directed by the anti-terrorist centre of the state security service. Forces from all the security units of the country have been brought in," Interfax news agency quoted the interior minister, Arsen Avakov, as saying on his Facebook page on Sunday.
One Ukrainian security officer has been killed, and five others injured since the operation began early on Sunday. At least one pro-Russian activist was also killed, according to Reuters news agency.
However, details of the fighting remain sketchy. A statement from the administration of the eastern Donetsk region indicated the security officer may have been killed between Slovyansk and the nearby town of Artemivsk.
Putting the number of wounded at nine, it said "an armed confrontation" was going on in the area.
Pro-Russian activists carrying automatic weapons seized government buildings in the town about 150km from the Russian border on Saturday, and set up barricades on the outskirts of Slovyansk.
Avakov also reportedly warned residents of the city to stay indoors, in anticipation of clashes between the armed men and Ukrainian security forces.
"Pass it on to all civilians: they should leave the centre of town, not come out of their apartments, and not go near the windows," Avakov was quoted as saying.
Ukraine accuses Russia of trying to deepen violence and chaos in Ukraine. The Kremlin, it says, wants to undermine presidential elections on May 25 which aim to set the country back on a normal path after months of turmoil.
However, Sergey Lavrov, the Russian foreign minister, said Kiev was "demonstrating its inability to take responsibility for the fate of the country".
He gave warning that any use of force against Russian speakers "would undermine the potential for co-operation", including talks due to be held on Thursday between Russia, Ukraine, the US and the EU.