Pro-Russian protesters who seized the regional government building in the Ukrainian city of Donetsk are reported to have declared a “people’s republic”.
The rebels have called for a referendum on secession from Ukraine by 11 May.
Ukrainian security officials are being sent to the eastern cities of Donetsk, Luhansk and Kharkiv after pro-Russia groups occupied government buildings.
Interim President Oleksandr Turchynov called the unrest an attempt by Russia to “dismember” Ukraine.
At the sceneSteve RosenbergBBC News, Donetsk
When I was in Donetsk three weeks ago, the regional administration building reminded me of a fortress: it was protected by a ring of riot police, barbed wire and water cannon parked in the yard.
Looking at the building this evening, the change couldn’t be more dramatic. The police have disappeared. In their place, pro-Russia activists are chanting “Russia! Russia!” Russian flags are flying from flagpoles outside.
The number of protesters on the square isn’t large: 1,000 at most. And it was only a few hundred who stormed the building last night. What’s more, surveys show that separatist sentiment in Donetsk and other parts of eastern Ukraine is not strong.
But small numbers can achieve big things when there is a power vacuum. And so far, the pro-Kiev authorities in Donetsk appear unable to restore order.
In an address on national TV, he said it was “the second wave” of a Russian operation to destabilise Ukraine, overthrow the government and disrupt planned elections.
Russia’s foreign ministry accused Kiev of “blaming” Moscow for all its troubles.
But the US later called on Russia’s President Vladimir Putin to stop “destabilising” Ukraine.
White House spokesman Jay Carney said escalating events were “the result of increasing Russian pressure”.