Border guard spokesman Sergei Astakhov said the Russians were demanding that Ukrainian soldiers and guards transfer their allegiance to Crimea's new pro-Russian local government.
"The Russians are behaving very aggressively, they came in by breaking down doors, knocking out windows, cutting off every communication," he said.
He said four Russian military ships, 13 helicopters and 8 transport planes had arrived in Crimea in violation of agreements that permit Russian to keep its naval base at Sevastopol.
Now, fears in the Ukrainian capital of Kiev and beyond are that Russia might seek to expand its control by targeting and seizing other parts of Ukraine, especially in its pro-Russian east.
