Ukrainian politician and likely presidential candidate Vitali Klitschko has called for a "general mobilisation" following the Russian parliament's decision to approve deploying troops in Ukraine's Crimea region.
In a statement Klitschko said: "Parliament must ask the army's commander-in-chief to declare national mobilisation after the start of Russian aggression against Ukraine."
He also asked for the UN Security Council to gather urgently for talks on the crisis.
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.