Secretary of State John Kerry has condemned Russia's "incredible act of aggression" and warned Moscow that it faced "very serious repercussions from the US and West, including sanctions that would isolate them economically."
On CBS' show 'Face the Nation', Mr Kerry added that Russia was behaving "in a 19th century fashion by invading another country completely on trumped up pretext."
He said G8 nations and some other countries are "prepared to go to the hilt and isolate Russia."
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.