U.S. Vice President Joe Biden has warned that the United States could impose tougher sanctions on Russia if it undermines Ukraine's presidential elections on May 25.
Biden Warns Russia of Tougher Sanctions if Ukraine Polls Undermined

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Ukrainian businessman, politician and presidential candidate Petro Poroshenko (right) meets supporters during his election campaign, in Odessa, May 21, 2014.

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Former Ukrainian prime minister and current presidential candidate Yulia Tymoshenko (right) meets supporters during her election campaign, Konotop, May 21, 2014.

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Ukrainian businessman, politician and presidential candidate Petro Poroshenko (left) meets supporters during his election campaign, in Odessa, May 21, 2014.

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Les manifestants scandent des slogans contre la garde présidentielle à Ouagadougou, au Burkina Faso, 16 Septembre, 2015.