CHONGAR, Ukraine—Crimean special forces and local militiamen with Kalashnikovs and masks have hoisted Russian flags and set up checkpoints on the only two highways that connect the Black Sea peninsula to mainland Ukraine.

The men are checking cars traveling in and out of Crimea, a region in Ukraine's south that was once part of Russia and has become the epicenter of a backlash against the pro-Western uprising in Kiev that has toppled President Viktor Yanukovych.

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