European politicians scrambled to throw their weight behind the parlous Kiev government in Ukraine .

British Foreign Secretary William Hague said the world supports Ukraine and will not leave it to fend for itself.

"What is taking place in Crimea now is a violation of the sovereignty and territorial integrity of Ukraine. The world will not abandon Ukraine," he said.

French Foreign Minister Laurent Fabius condemned Russia's actions as unacceptable, saying: "The Russians ... want to get their hands back on Crimea. Obviously, as far as international law is concerned, it is unacceptable."

And US Assistant Secretary of State Victoria Nuland said: "We are using all of our channels to make the case to Russia ... that it should settle its issues politically, not with military force."

New Prime Minister Arseniy Yatsenyuk claimed any attempt to seize Crimea would fail.

However, he also said that "for today, no military options are on the table," pleading instead for international economic and political support.

But Russia has vowed its troops will remain in Ukraine to protect Russian interests and citizens until the political situation is "normalised."

And saw unverified reports that Russia's Black Sea Fleet had given Ukrainian forces until 3am today to surrender or face all-out assault.

It has become clear that Russia is now in military control of Crimea.

There are also demonstrations in eastern Ukraine in which Moscow's intervention is being warmly welcomed.

Some 2,000 people waving Russian flags rallied outside the regional government building in Donetsk to protest at the appointment of a new pro-Kiev governor and dozens later occupied the first floor of the building.

Two large Ukrainian military bases are surrounded with Russian troops standing alongside local self-defence groups, who are calling on the Ukrainian soldiers inside to defect to Crimea's new government.