Ukraine's Acting President Oleksander Turchinov has vowed to launch a full-scale anti-terrorist operation utilising the army after pro-Russian separatists seized government buildings.
He was angered by the death of a state security officer and said he would not allow a repetition of what happened in Crimea which was annexed by Russia last month.
"The blood of Ukrainian heroes has been shed in a war which the Russian Federation is waging against Ukraine," Turchinov said in a televised address to the nation. "The aggressor has not stopped and is continuing to sow disorder in the east of the country."
Russia's foreign ministry called the planned military operation a "criminal order".
"It is now the West's responsibility to prevent civil war in Ukraine," the ministry said in a statement.
Earlier yesterday, Ukrainian security forces launched an operation yesterday to clear pro-Russian separatists from state buildings in the eastern city of Slaviansk, with deaths reported on both sides as Kiev combats what it calls "an act of aggression by Moscow".
Nato described the appearance in eastern Ukraine of men with specialised Russian weapons and identical uniforms without insignia - as worn by Moscow's troops when they seized Crimea - as a "grave development".
Moscow has warned it will protect the region's Russian-speakers if they come under attack.
The Russian news agency RIA reported that one pro-Moscow activist was killed in Slaviansk in clashes with forces loyal to the Kiev government. "On our side, another two were injured," RIA quoted pro-Russian militant Nikolai Solntsev as adding.
The separatists are holed up in the headquarters of the police and of the state security service, while others have erected road blocks around Slaviansk, which lies 150km (90 miles) from the Russian border.
But 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 Slaviansk and the nearby town of Artemivsk. Putting the number of wounded at nine, it said "an armed confrontation" was going on in the area.
Kiev accuses Moscow of trying to deepen violence and chaos in Ukraine. The Kremlin, it says, wants to undermine the legitimacy of presidential elections on May 25.
However, Russian Foreign Minister Sergei Lavrov warned that any use of force against Russian speakers "would undermine the potential for cooperation", including talks due to be held on Thursday between Russia, Ukraine, the US and the EU.
Relations between Russia and the West are at their worst since the Cold War due to the crisis that began when Moscow-backed Ukrainian president Viktor Yanukovich was pushed out by popular protests in February.
Moscow then annexed Crimea from Ukraine, saying the Russian population there was under threat. Some Western governments believe the Kremlin is preparing a similar scenario for eastern Ukraine, something Moscow has denied.
In Kramatorsk, about 15km south of Slaviansk, gunmen seized the police headquarters after a shootout with police, a Reuters witness said. The attackers were a well-organised unit of more than 20 men, wearing matching military fatigues and carrying automatic weapons, who had arrived by bus.
Video footage showed the men taking orders from a commander.
Their level of discipline and equipment was in contrast to the groups who have occupied buildings so far in Ukraine. They have been mostly civilians formed into informal militias with mismatched uniforms.
Nato Secretary-General Anders Fogh Rasmussen expressed concern about similarities in some of the separatists' appearance to that of the Russian troops who seized control in Crimea.