The U.S. kept up a war of words with Russian President Vladimir Putin while hoping he will back down over Ukraine, but there was little evidence Tuesday he would.

Mr. Putin offered a full-throated defense of his use of force in the restive region of Crimea, rejecting Western demands to withdraw and insisting sanctions would be counterproductive. Russian troops occupying an air base there fired warning shots at Ukrainian counterparts, seemingly underlining Moscow's determination.

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