The activists in Crimea raised the Russian flag over the navy base in Sevastopol, the BBC reported. Ukrainian navy leader Serhiy Hayduk was being held and some Ukrainian military personnel were observed leaving the base, at least one in tears, the British broadcaster reported.
Andriy Parubiy, secretary of Ukraine's national security agency, responded with a statement saying the country would seek action from the United Nations, the Interfax news agency reported. Acting President Rada Oleksandr Turchynov said the country will take "appropriate steps" to respond unless all hostages in Crimea were released immediately.
Ukrainian Defense Minister Ihor Tenyukh and First Deputy Prime Minister Vitaly Yarema went to Crimea Wednesday to try to ease tensions, but were barred from entering the peninsula, Ukrainian media reported citing ministers.
Russia's Constitutional Court recognized the treaty on Ukraine's breakaway Crimea's reunification with Russia, the court's chairman said Wednesday. Parliamentary approval was expected to quickly follow.
Russian President Vladimir Putin Tuesday approved the draft treaty on Crimea's reunification with Russia and the formation of new regional authorities as part of the Russian Federation, ITAR-Tass reported.
"The Constitutional Court recognizes that the Treaty corresponds to the Russian Constitution," Valery Zorkin, the Constitutional Court chairman, said of the document, which went into effect when Putin signed it.
Voters in autonomous, pro-Russia Crimea overwhelmingly approved a referendum Sunday on breaking away from Ukraine and joining Russia.
Leaders of Crimea and Russia have refused to recognize the new government in Kiev that came to power after the ouster of pro-Russian President Viktor Yanukovych last month. Crimea declared its independence and said it would seek reunification with Russia, which sent thousands of troops to the peninsula, despite Putin's denials.
Yanukovych, who maintains he's Ukraine's legitimate leader, surfaced in Russia soon after he was ousted.
Crimea was part of Russia until it was given to Ukraine by former Soviet Union leaders in 1954.