About 93% of voters in Ukraine's Crimea region supported union with Russia, Russian state news agency RIA quoted an exit poll as showing.
Polling stations in Crimea closed at 6pm GMT. Another Russian news agency, Interfax, said voter turnout had exceeded 80%.
The exit poll cited by RIA was conducted by the Crimean Institute of Political and Social Research.
Putin's decree recognising Crimea as a 'sovereign country' has triggered the toughest sanctions against Russia since the Cold War.
With over half of the votes counted, over 95% of people in Crimea have voted to become part of Russia, election officials have said.
At some point the West's warnings will have to become deeds, part of the problem is they don't know which Putin they're dealing with.