Russia has vowed its troops will remain in Ukraine to protect Russian interests and citizens until the political situation has been "normalised".
Foreign Minister Sergei Lavrov said Russia was defending human rights against "ultra-nationalist threats".
Russia is now in de facto military control of the Crimea region, despite Western condemnation of a "violation of Ukraine's sovereignty".
Ukraine has ordered full mobilisation to counter the military intervention.
Mr Lavrov said in Geneva on Monday that Russian troops were needed in Ukraine "until the normalisation of the political situation".
Russia's parliament authorised the use of troops on Saturday, in the wake of the ousting of pro-Russian President Viktor Yanukovych last month.
He said the "violence of ultra-nationalists threatens the lives and the regional interests of Russians and the Russian speaking population".
Mr Lavrov, who will meet UN Secretary General Ban Ki-moon in Geneva later, also condemned Western threats of sanctions and boycotts.
The BBC's Imogen Foulkes, in Geneva, says the comments were in stark contrast to those in a text previously distributed to journalists, in which Mr Lavrov said that "military interventions on the pretext of civilian protection produce the opposite effect".
In Kiev, Ukraine's Prime Minister Arseniy Yatsenyuk said that any attempt to seize Crimea would fail.
However, he also said that "for today, no military options are on the table", urging instead international economic and political support.
The crisis has hit Russian stock markets, with Moscow's main MICEX index dropping 9% in early trading. The rouble fell to a fresh all-time low against the US dollar and Russia's central bank raised its key lending rate to 7% from 5.5%.
The BBC's Mark Lowen in Sevastopol says Crimea is now under de facto Russian armed control although no shots have been fired.
He says two large Ukrainian military bases are surrounded and key installations like airports are occupied.
Thousands of newly arrived Russian elite troops far outnumber Ukraine's military presence, he says, with roadblocks cutting off Crimea.
Ukrainian border guards have reported a build-up of armoured vehicles on the Russian side of the sea channel dividing Russia and Crimea.