With Russian troops on the move in the Moscow-controlled Crimea, security remains a top concern.<br/> <br />So much so that international observers cut short their visit to monitor the situation on the ground.<br/> <br />(SOUNDBITE) (English) PERSONAL ENVOY OF THE SWISS OSCE CHAIR IN OFFICE, TIM GULDIMANN, SAYING<br/> <br />"It's extremely tense, and I would consider this as a miracle that bloodshed could be avoided so far, given the political and even military circumstances on the ground."<br/> <br />The delegation from the Organization for Security and Cooperation in Europe arrived in Crimea Wednesday.<br/> <br />But hostile crowds forced a UN envoy to abandon his mission to the region, with the delegation returning to Kiev.<br/> <br />Now there are concerns about the upcoming referendum.<br/> <br />SOUNDBITE) (English) OSCE HIGH COMMISSIONER ON NATIONAL MINORITIES, ASTRID THORS, SAYING:<br/> <br />"How legal is a referendum or a plebiscite taking into a situation where you have a strong military presence? We are not sure