Pro-Russian protesters occupying a government building in eastern Ukraine have declared the creation of a separatist republic.
The rebels have called for a referendum on secession from Ukraine by 11 May.
"In the event of aggressive action from the illegitimate Kiev authorities, we will appeal to the Russian Federation to bring in a peacekeeping contingent," said the proclamation, voiced by an unidentified bearded man.
The activists later read out the text by loud hailer to a cheering crowd of about 1,000 people outside the building.
Ukrainian security officials are being sent to the eastern cities of Donetsk, Luhansk and Kharkiv after pro-Russia groups occupied government buildings.
Ukraine\’s acting president has pledged "anti-terrorist measures" as the government declared the actions of pro-Russian protesters in the country east a "second stage" of special operations by Russia under way to break up the troubled state.
Oleksander Turchinov, in a televised appeal on Monday, said that the action in the cities of Kharkiv, Donetsk and Luhansk showed that Russia was "playing out the Crimean scenario" – a reference to the pro-Russian takeover of the peninsula and its annexation by Russia.
"We will not allow this," he said. Warning that some activists had taken up arms, Turchinov said: " Anti-terrorist measures will be carried out against those who had taken up weapons."
Ukraine\’s prime minister has accused Russia of backing separatist demonstrations, including the seizure of government buildings Sunday in three eastern Ukrainian cities.
Arseniy Yatsenyuk told a Cabinet meeting Monday that Russia is behind the unrest that rose up in the eastern cities of Donetsk, Luhansk and Kharkiv. He said the demonstrations are clearly part of a plan to destabilize the situation and allow "foreign" troops to cross the border and seize Ukrainian territory.
Yatsenyuk said Russian troops remain stationed within 30 kilometers of Ukraine.
Late on Monday, Russian foreign ministry issued a statement, saying: "Stop… blaiming all the troubles of today\’s Ukraine on it (Moscow)."
"If irresponsible treatment of one\’s country, one\’s nation, by the political forces who call themselves the Ukrainian authorities continues, Ukraine will inevitably be facing new troubles and crises," it said.
But US Secretary of State John Kerry said that the events "did not appear to be spontaneous".
He called on Russia to "publicly disavow the activities of separatists, saboteurs and provocateurs" in a phone call to his Russian counterpart Sergei Lavrov.
The pair also discussed convening direct talks between Ukraine, Russia, the US and the European Union within ten days, the US state department said.
In Luhansk, pro-Russia activists are reported to have seized facilities where weapons are stored. Ukrainian highway police have closed off roads into the city.
And in an eastern Crimea military dormitory, a Russian soldier shot dead a Ukrainian naval officer during an argument, as the Ukrainian was packing his belongings to leave the region.
Protesters broke into a government building in another eastern city, Kharkiv, on Sunday, but interior minister Arsen Avakov later said those protesters had been cleared from the premises. Like Ukraine\’s prime minister and president, he, too, blamed Russia for the unrest.
The protesters in Donetsk, Luhansk, and Kharkiv demanded the cities hold referendums on whether to split with Ukraine and become part of Russia, the same kind of referendum held last month in Crimea.
Ukrainians have been split between those loyal to the Kyiv government and those calling for stronger ties to Russia, following Russia\’s annexation of the Crimean peninsula last month.