Pro-Russian forces have taken control of two naval bases in Crimea – including the HQ of Ukraine’s navy.
Streams of Ukrainian servicemen were seen leaving the bases. Kiev said its navy chief has been detained.
Crimean leaders signed a treaty with Moscow on Tuesday absorbing the peninsula into Russia.
Sunday’s referendum approving Crimea’s split from Ukraine came nearly a month after Ukraine’s pro-Russian President Viktor Yanukovych fell from power.
Crimean and Russian officials said the vote showed overwhelming public support for joining Russia, with 97% of voters in favour.
But the West and the Ukrainian government in Kiev say the hastily organised referendum – boycotted by many of Crimea’s Ukrainian and Tatar minorities – was illegal and will not be recognised.