Ministers scrambled to meet while the world watched with alarm as Russian President Vladimir Putin won approval from parliament's upper house to use Russian troops in Ukraine and Kiev accused Moscow of sending thousands of soldiers into Crimea.
"Ashton calls extraordinary Foreign Affairs Council on developments in Ukraine. Monday, 3 March. Meeting starts 1300 CET," she said on Twitter.
Though the Kremlin said Putin had not yet decided to deploy the troops, Ukrainian leaders suggested a "national mobilisation" and the UN Security Council called emergency talks.
"We mustn't forget the Caucasus was a powder-keg in the past. That's why Europe must speak with a single voice and put an end to the blundering."
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Poland's prime minister Donald Tusk urged unity, saying Europe needed to give a "clear signal" that no act of aggression would be tolerated.
Tusk said that not everyone in the European Union had "realised the gravity of the situation and the risks that Europe and this region are facing".
Sweden's Foreign Minister said on Twitter that the situation was the "worst European crisis since a long time back. We need a strong EU in an unstable Europe."
Ukraine's parliament ousted Yanukovych on February 22.
The sanctions have not yet been detailed or imposed, but in London British Foreign Secretary William Hague said "the EU must agree urgently an asset freezing regime to target those suspected of laundering the proceeds of corruption."
EU diplomats speaking on condition of anonymity have said an assets freeze and visa ban is likely against several top officials behind the deadly use of force against protesters.
Switzerland and Austria have already taken similar action.