A convoy said to be carrying hundreds of Russian soldiers has moved towards a base near the regional Crimean capital Simferopol.
Tensions have risen in the peninsula since the region’s pro-Moscow leadership declared it part of Russia and announced a referendum.
Meanwhile, warning shots have been fired to stop an unarmed international military observer mission from entering Crimea.
The 40-strong group, from the Organisation for Security and Cooperation in Europe, has now been turned back three days in a row.
In Simferopol, a military ceremony has been held, creating what’s described as a Crimean defence force.
Around 30 uniformed and armed men were presented as the “First Division” at the event – attended by Crimea’s self-proclaimed prime minister.
It came after troops, believed to be Russian, drove a truck into a missile defence post in Sevastopol – the home of both their Black Sea Fleet and the Ukrainian navy.
Ukraine’s border service has said that Russian troops also seized a border guard outpost in the east of the peninsula.