Pro-Ukrainian supporters take part in a rally in Simferopol March 9, 2014. Russian forces tightened their grip on Crimea on Sunday despite a U.S. warning to Moscow that annexing the southern Ukrainian region would close the door to diplomacy in a tense East-West standoff. The message on the balloon reads: "We are for peace in Ukraine." REUTERS/Thomas Peter (UKRAINE - Tags: POLITICS CIVIL UNREST)