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"We are calling for calm and we are calling for all actors to try to ratchet down the tensions, that's the overall message," UNHCR spokesperson Matthew Saltmarsh told a U.N. briefing.

"In terms of contingency plans, we've been talking to partners, we have the ability to move in supplies and stuff if needed," he added.

Tensions have escalated after Russia massed an estimated 100,000 troops within reach of the Ukrainian border, although Moscow denies any intention of invading.