In what some call "crunch time" others a worst case scenario, Ukraine Announces Full-Scale Army Operation on Pro-Russian Militants in Seized Buildings.
Ukraine's president says a full-scale operation involving the army will be launched in the east after pro-Russian militants seized government buildings.
Acting President Oleksandr Turchynov said he would not allow a repetition of what happened in Crimea which was annexed by Russia last month.
His live televised address from parliament came after pro-Russian forces targeted half a dozen cities.
An urgent meeting of the UN Security Council will be held in New York.
The meeting, at 20:00 New York time (midnight GMT), was called for by Moscow, which has strongly criticised Kiev's plan to use its armed forces in eastern Ukraine.
Responding to Mr Turchynov's address, a Russian foreign ministry spokesman said the plan to use the armed forces was "criminal" and caused "particular indignation".
Ukraine was, the spokesman said, "waging war against its own people".
Nato chief Anders Fogh Rasmussen's statement on Sunday drew parallels with some aspects of last month's seizure of Crimea.
He said the "reappearance of men with specialised Russian weapons and identical uniforms without insignia, as previously worn by Russian troops during Russia's illegal and illegitimate seizure of Crimea, is a grave development".
On Saturday, armed men took over police stations and official buildings in Sloviansk, Kramatorsk and Druzhkivka.
Similar accounts emerged of armed men dressed in camouflage arriving in buses in Sloviansk and Kramatorsk and storming the police stations.
BBC reporters in Sloviansk said the gunmen were well-organised and quickly established control throughout the town. Checkpoints had been set up on the main roads into the town.