Ukraine put its armed forces on full combat alert and warned Russia that any military intervention in the country would lead to war.
After a more than three-hour meeting with security and defence chiefs, Acting President Oleksander Turchinov said there was no justification for what he called Russian aggression against his country.
Standing beside Turchinov, Prime Minister Arseny Yatseniuk said he had urged Russia to return its troops to base in the Crimea region during a phone call with Prime Minister Dmitry Medvedev and called for talks.
"Military intervention would be the beginning of war and the end of any relations between Ukraine and Russia," Yatseniuk told reporters.
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.