Ukraine has issued an ultimatum to pro-Russian forces to release navy chief Serhiy Hayduk and all hostages captured when naval headquarters in Sevastopol were stormed.
Ukraine's interim president Olexander Turchynov warned Crimean authorities they had three hours to free Hayduk and other hostages, Unian news agency reported.
"The government will take adequate steps unless all provocations against Ukrainian servicemen end, and Admiral Hayduk and other hostages - both military and civilian - are freed," Turchynov said.
"Russian soldiers are cynically refusing to hold negotiations to free hostages."
Attackers burst through the base gates using an ambulance car. They met no armed opposition.
"There is nothing we could do against the crowd, nothing. Everything happened spontaneously," Captain Olexander Balanyuk told the BBC.
"There were many promises from the Russian side and our side that the base will not be stormed, that all issues will be resolved through political means, but as you see now - there was a takeover."
Ukrainian servicemen at first barricaded themselves inside a building; many were later allowed to leave the base to the cheering of the invaders.
A spokesman for the pro-Russian forces claimed Hayduk had been captured and taken away by the Russian Federal Security Service.
"He was blocked and he had nowhere to go. He was forced out and he has been taken away," Igor Yeskin said.
Kiev said it had begun drawing up plans to pull its troops from the southern peninsula.
National Security and Defence Council secretary Andriy Parubiy said Ukraine was seeking to relocate its armed forces to the mainland.
"We are developing a plan that would enable us not only to withdraw servicemen, but also members of their families in Crimea, so that they could be quickly and efficiently moved to mainland Ukraine," Parubiy said.