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Crimeans head to the polls Sunday to vote in a referendum on whether the peninsula should break away from Ukraine and join Russia. The United States, its allies, and the interim Ukrainian government have assailed the vote as a Russian stunt with no constitutional legitimacy. The Group of 7 last week said in a statement the referendum was "contrary to Ukrainian law and in violation of international law," adding that it "would have no legal effect."

However, Russia on Saturday vetoed a proposed U.N. resolution that would have preemptively declared the referendum's results invalid. Officials expect Crimea will resoundingly vote to split from Ukraine, delivering a victory to Moscow, which sent troops into the region earlier this month.

"The result has been pre-planned by the Kremlin as a formal justification to send in its troops and start a war that will destroy people's lives and the economic prospects for Crimea," Oleksandr Turchynov, Ukraine's interim president, said.