Ukraine is calling up military reservists following Russia’s decision to deploy troops in Crimea.
Ukraine’s acting President Oleksandr Turchynov had already ordered increased security at key sites, including nuclear plants.
Meanwhile President Barack Obama has called the Russian decision a “violation of Ukrainian sovereignty”.
Ukraine has said it will seek the help of US and UK leaders in guaranteeing its security. NATO has called emergency talks to be held on Sunday at 12:00 GMT.
The new Ukrainian PM Arseniy Yatsenyuk has warned that military action by Moscow would be “the beginning of war and the end of relations”.
Heavily armed groups continue to occupy key sites in Crimea, including airports and communications hubs, although there has been no actual violence.
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