The Russian foreign ministry has said that Russia is "extremely concerned" about developments in the Crimea region of Ukraine.
The ministry added the developments confirmed the desire of Kiev's politicians to destabilise the situation on the peninsula.
"We believe it is extremely irresponsible to further pressure the already tense situation in the Crimea," the foreign ministry said in a statement.
Earlier today Ukraine's Defence Minister said Russia has recently brought 6,000 additional troops into the country.
Pro-Russia activists recaptured administrative headquarters in the Ukrainian city of Donetsk and flew the Russian flag from its roof.
ITV News' Europe Correspondent describes the tense atmosphere surrounds a central government building in Donetsk .
New footage obtained by ITV News shows Russian troops firing warning shots as Ukrainian soldiers approached during a tense stand-off today.