President Oleksandr Turchynov said: "Any movements of troops, especially with troops outside that territory will be considered military aggression."
Russia maintains a large naval base in southern Crimea that has strained relations between the countries for two decades.
Ukraine put its police on high alert after dozens of armed pro-Russia men stormed and seized local government buildings in Ukraine's Crimea region early today and raised a Russian flag over a barricade.
The renewed tension in this strategic peninsula that houses Russia's Black Sea fleet comes as lawmakers in Kiev were expected to approve the new government in the wake of the president fleeing the capital after months of protests over his decision to scuttle an agreement with the European Union in favour of stronger ties with Russia.
Live: BSE Sensex