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 Ukrainian region of Crimea, and hoisted Russian flags.
It is also being reported the deposed Ukrainian President Viktor Yanukovych has been spotted in Moscow, and is being put up in a Kremlin property outside the city.
Insisting he is still president, Mr Yanukovych is said to be seeking a guarantee for his safety from the actions of “extremists”.
The dawn raid in the regional 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”.
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 Turchinov 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 failing to do so “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”.
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, 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”.
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.
He is now a fugitive, wanted in connection with the deaths of around 100 people during the recent violence.
Ukraine’s new government is expected to be formally approved by parliament on Thursday, and faces the hugely complex task of restoring stability in the country.