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
![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](https://gdb.voanews.com/fe7a694e-266d-498b-a85b-30fa5761476f_w1024_q10_s.jpg)
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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
![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)](https://gdb.voanews.com/036bd9be-25f7-422d-b156-37e66a6be80d_w1024_q10_s.jpg)
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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)
![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](https://gdb.voanews.com/d24f8ca9-8baa-4b0b-b689-4e88296ad597_w1024_q10_s.jpg)
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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
![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](https://gdb.voanews.com/45a4216b-d3a8-4fb6-af90-307fdbafae7a_w1024_q10_s.jpg)
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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