2 March 2014 Last updated at 18:34 GMT
Admiral Berezovsky read out his oath in Sevastopol alongside Crimea's pro-Russian leader, Sergiy Aksyonov
The newly appointed head of Ukraine's navy has sworn allegiance to the Crimea region, in the presence of its unrecognised pro-Russian leader.
Rear Admiral Denys Berezovsky was only made head of the navy on Saturday, as the government in Kiev reacted to the threat of Russian invasion.
Russia's troops have been consolidating their hold on Crimea, which is home to its Black Sea Fleet.
The US has warned Moscow may be ejected from the G8 for its actions.
US President Barack Obama called Russian troop deployments a "violation of Ukrainian sovereignty".
'Brink of disaster'
Ukraine has ordered a full military mobilisation in response to Russia's build-up of its forces on the Crimean peninsula. Prime Minister Arseniy Yatsenyuk has warned the country is "on the brink of disaster".
In Crimea, Ukrainian soldiers faced off with Russian soldiers surrounding their bases on Sunday while the Russian army was said to be digging trenches on the border with mainland Ukraine.
So in Ukraine is that what Russia intends to do too? Take Crimea and Russian speaking regions under its control, effectively dividing Ukraine in half?
Bridget Kendall
Diplomatic correspondent, BBC News
The UK has joined the US, France and Canada in suspending preparations for a summit of the G8 in Russia in June. Nato, of which Ukraine is not a member, is conducting emergency talks.
Admiral Berezovsky appeared in Sevastopol before cameras alongside Sergiy Aksyonov, the pro-Russian politician elected by Crimea's regional parliament as local prime minister.
Mr Aksyonov announced he had given orders to Ukrainian naval forces on the peninsula to disregard any orders from the "self-proclaimed" authorities in Kiev.
Sunday, he said, would go down in history as the birthday of the "navy of the autonomous republic of Crimea".
Admiral Yuri Ilyn: "I'm very sorry that Ukrainian soldiers and sailors are hostages of this situation"
The admiral then pledged to "strictly obey the orders of the supreme commander of the autonomous republic of Crimea" and "defend the lives and freedom" of Crimea's people.
Admiral Berezovsky was later sacked by interim Ukrainian Defence Minister Ihor Tenyukh and a treason case launched against him.
Ukraine's Ukrainska Pravda newspaper reports that the admiral was speaking before the "numerous cameras of predominantly Russian TV channels".
Earlier, Ukrainian naval officers found their headquarters in Sevastopol occupied by Russian troops and were unable to go to work.
Admiral Yuriy Ilyn, who was until recently commander of the Ukrainian navy and served briefly as head of Ukraine's armed forces under ousted President Viktor Yanukovych, told the BBC's Christian Fraser at the scene that the armed forces were "hostages of the situation".
John Kerry: Russian troop movement is a "brazen act of aggression"
Separately, Ukraine withdrew coast guard vessels from two ports in Crimea and moved them to other bases in the Black Sea and Sea of Azov on Sunday.
Several Ukrainian army bases were surrounded by Russian troops on Sunday but there were no reports of clashes despite the refusal of Ukrainian soldiers to open their gates.
John Kerry told US media Russia might "not even remain in the G8" and Russian President Vladimir Putin might "find himself with asset freezes".
"You just don't in the 21st-Century behave in 19th-Century fashion by invading another country on completely trumped-up pretext," Mr Kerry told the CBS program Face the Nation.
The Russian parliament authorised Mr Putin on Saturday to deploy troops in Ukraine to protect the lives of Russian citizens there.
Moscow has not recognised the government which took power in Kiev last month after overthrowing the elected pro-Russian President, Viktor Yanukovych.
Mr Yanukovych's decision in November to abandon closer ties with the EU in favour of Russia sparked massive protests in Kiev, which ended in a bloodbath, as dozens of protesters were shot dead in clashes with police.
