Ukraine has ordered a full military mobilisation in response to Russia's build-up of its forces in Crimea.
Prime Minster Arseniy Yatsenyuk said the country was "on the brink of disaster".
US President Barack Obama has called Russian troop deployments a "violation of Ukrainian sovereignty".
Nato is conducting emergency talks on the crisis. Its secretary-general has said Russia's actions "threatened peace and security in Europe".
Several other measures were announced on Sunday by national security officials:
The armed forces would be put on "full combat readiness".
- Reserves to be mobilised and trained
- Ukraine's foreign minister will seek the help of US and UK leaders in guaranteeing its security
- Emergency headquarters to be set up
- Increased security at key sites, including nuclear plants.
- Airspace closed to all non-civilian aircraft.
The BBC has seen what appear to be Russian troops digging trenches on the Crimean border.
02 March 2014
The BBC reports on the latest in the mobilization in Ukraine.