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.
ພາບຫຼ້າສຸດ ຈາກ ຢູເຄຣນ

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Ukrainian businessman, politician and presidential candidate Petro Poroshenko (L) meets supporters during his election campaign in Odessa May 21, 2014. REUTERS/Mykola Lazarenko/Pool (UKRAINE - Tags: BUSINESS POLITICS ELECTIONS) - RTR3Q6BH

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Local residents walk with their young children in a park in Donetsk, Ukraine, on Wednesday, May 21, 2014. Ukraine are hold a presidential election on Sunday(AP Photo/Evgeniy Maloletka)

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Workers stand near pipes at a gas compressor station near Uzhhorod May 21, 2014. Russia has said state-controlled exporter Gazprom will not supply transit nation Ukraine with gas for its own use in June if Kiev fails to pay in advance and has warned a cut

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Ukrainian businessman, politician and presidential candidate Petro Poroshenko (R) meets supporters during his election campaign in Odessa May 21, 2014. REUTERS/Mykola Lazarenko/Pool (UKRAINE - Tags: POLITICS ELECTIONS BUSINESS) - RTR3Q6BD