Biden Warns Russia of Tougher Sanctions if Ukraine Polls Undermined

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

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

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

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

Local residents walk with their young children in a park, in Donetsk, May 21, 2014.

Workers stand near pipes at a gas compressor station near Uzhhorod. Russia has said state-controlled exporter Gazprom will not supply Ukraine with gas in June if Kyiv fails to pay in advance, May 21, 2014.