Andriy Paruby, Ukraine's top security official, accused the Kremlin of commanding armed groups in Crimea where two airports have been taken over by armed men.
"These are separate groups ... commanded by the Kremlin," Paruby, secretary of the National Security and Defence Council, told a televised briefing in Kiev.
He said similar actions had been prevented elsewhere in Ukraine. He said Ukraine could not deploy military forces in Crimea without introducing a state of emergency.
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.