Russia has tightened its military grip on Ukraine's Crimea region, taking de facto control despite Western demands that it withdraw.
Thousands of Russian troops are securing the region and further armour and ship movements have been reported.
The world's seven major industrialised powers have condemned Moscow's 'violation of Ukraine's sovereignty'.
Ukraine has ordered full mobilisation, issuing call-up papers and asking for more international support.
Russia says it is protecting its interests and those of Russian-speakers in Crimea and elsewhere in Ukraine following the ousting of pro-Russian President Viktor Yanukovych last month.
The crisis hit Russian stock markets on Monday, 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 Crime.