Sorry for the source on this, I could not find it elsewhere. The Ukrainian statement about this this blockade is that they are worried about saboteurs infiltrating.
Ukraine has decided to suspend all rail and bus services to Crimea over what it calls security concerns.
The decision came days after Russia decided to halt its train service to Crimea due to low demand. The Black Sea peninsula has no overland connection to Russia and relies on a land bridge to Ukraine for basic needs. Now, most of the region’s residents will have to reach Ukraine by car or cross into Russia by ferry. Earlier this week, Kiev temporarily cut power supplies to Crimea, saying the Russian-controlled peninsula had exceeded consumption limits. In March, Crimeans voted to break away from Ukraine and rejoined Russia in a referendum that angered Kiev and its Western allies.