By Marie-Louise Gumuchian and Susanna Capelouto, CNN
updated 1:28 PM EST, Mon March 3, 2014

(CNN) -- Ukrainian officials say Aleksandr Vitko, the Russian commander of the Black Sea Fleet, went aboard a blocked Ukrainian warship in Crimea's Sevastopol Harbor on Monday and issued a threat:

"Swear allegiance to the new Crimean authorities, or surrender, or face an attack," he said, according to Vladislav Seleznev, a Ukrainian Defense Ministry spokesman in Crimea who spoke to CNN. He said the Russian commander did not mention an ultimatum deadline.

But a spokesman for the Russian Black Sea Fleet said there are no plans to storm Ukrainian military units in Crimea, according to the Interfax news agency.

The European Union foreign affairs council strongly condemned what it called Russia's "clear violation of Ukrainian sovereignty and territorial integrity" and demanded that Russia pull its forces back to their barracks.