Fighter jets have been put on combat alert by Russia along its border with Ukraine as it pledged to defend the rights of its "compatriots" in the strife-torn country.
It came amid escalating tensions after armed men seized the parliament building and the regional government headquarters in the southern Ukrainian region of Crimea, and hoisted Russian flags.
Meanwhile in the capital Kiev, Ukraine's parliament formally approved the formation of a national coalition government, which faces the hugely complex task of restoring stability in the country.
Police outside the parliament building after it was stormed by armed men
It is also being reported that deposed Ukrainian President Viktor Yanukovych has been spotted in a luxury five-star hotel and spa outside Moscow, in an exclusive enclave favoured by Russia's super-rich.
Mr Yanukovych, who was driven out of office after three months of protests which spiralled into bloody violence, insists he is still president.
Russia has granted protection to the 63-year-old fugitive, who is wanted in Ukraine on charges of mass murder after police opened fire on demonstrators.
The dawn raid in the Crimean capital of Simferopol saw up to 50 gunmen in combat fatigues storm the official buildings, erect barricades, and put up signs saying 'Crimea is Russian'.
The parliament building has been seized in Simferopol
They are also reported to be wearing black and orange ribbons, a Russian symbol of victory in World War Two.
Ukraine's acting president Oleksandr Turchynov has appealed for calm, and branded the armed raiders as "criminals in military fatigues".
He also urged Russian forces not to leave their base in southern Crimea, warning that it "would be considered a military aggression".
Ukraine's interim interior minister Arsen Avakov said security forces had been put on alert in Simferopol and the buildings cordoned off "to prevent bloodshed".
It has led Poland's Foreign Minister Radoslaw Sikorski to warn of a "very dangerous game" in Crimea, amid concerns it could trigger a regional conflict.
Barricades have been erected by the gunmen
Crimea is an autonomous republic in the south of Ukraine, around 500 miles from the capital Kiev.
There are mounting signs of separatism in the region which has strong ties to Moscow, and where the majority of the population are Russian speakers.
On Wednesday there was a confrontation between pro-Russia separatists and ethnic Tatars, who support Ukraine's new leaders, outside the parliament building.
Russia has questioned the legitimacy of the new Ukrainian authorities, and has pledged to defend the rights of its "compatriots".
It would react "uncompromisingly" if these were violated, the foreign ministry said in a statement.
President Vladimir Putin has put the military on alert for major exercises
President Vladimir Putin has put the military on alert for major exercises, and announced measures to tighten security at the headquarters of Russia's Black Sea Fleet in Crimea.
The move prompted a rebuke from US Secretary of State John Kerry, who warned Russia against any military intervention in Ukraine.
Stressing the Ukraine crisis should not be seen in a Cold War context, he said: "I want to underscore to everybody that this is not Rocky 4. It is not a zero-sum game."
Nato Secretary-General Anders Fogh Rasmussen has also said he was concerned about developments in Ukraine's Crimea, and urged Russia not to do anything that would "escalate tension or create misunderstanding".
Russia has been warned against any military intervention over Ukraine
Ukraine's newly appointed Prime Minister Arseny Yatseniuk, who was a prominent protest leader, has insisted the country would not accept the southern Crimea region breaking away.
Crimea "has been and will be a part of Ukraine," he said.
He also revealed that in the last three years about $70bn had been siphoned out of Ukraine's financial system into offshore accounts.
It came as Russia confirmed it will take part in talks on an International Monetary Fund (IMF) financial package for Ukraine, where the economic situation continues to worsen after months of unrest.
Debt-hit Ukraine has said it needs $35bn (£21bn) over the next two years to avoid bankruptcy.
The European Commission says Russia should be included in efforts to provide financial assistance.
Ukraine has also seen continuous devaluation of its currency amid the political upheaval, which led to Mr Yanukovych being toppled.
Mr Yatseniuk said securing a new loan agreement with the IMF was vital for the country to be able to stabilise the currency.
He also said tough reforms are needed to prevent Ukraine, divided in loyalties between Russia and the West, from collapsing economically and politically.
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