With Russian troops on the move in the Moscow-controlled Crimea, security remains a top concern.

So much so that international observers cut short their visit to monitor the situation on the ground.

(SOUNDBITE) (English) PERSONAL ENVOY OF THE SWISS OSCE CHAIR IN OFFICE, TIM GULDIMANN, SAYING

"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."

The delegation from the Organization for Security and Cooperation in Europe arrived in Crimea Wednesday.

But hostile crowds forced a UN envoy to abandon his mission to the region, with the delegation returning to Kiev.

Now there are concerns about the upcoming referendum.

SOUNDBITE) (English) OSCE HIGH COMMISSIONER ON NATIONAL MINORITIES, ASTRID THORS, SAYING:

"How legal is a referendum or a plebiscite taking into a situation where you have a strong military presence? We are not sure