Pro-Russian forces have taken control of Ukraine's navy headquarters in the Crimean port of Sevastopol, a day after Russia signed a treaty with local authorities to make Crimea part of Russia.
Witnesses say at least 200 unarmed self-defense forces entered the base Wednesday and raised the Russian flag. Ukrainian service members did not resist and were seen leaving the facility.
Russian news agencies quoted sources in the Sevastopol prosecutor's office as saying the commander of Ukraine's navy, Sergei Gaiduk, had been "temporarily detained" after leaving the navy headquarters.
And later Wednesday, reports said a second naval base had been seized by Russian forces.
Tuesday, a Ukrainian serviceman and a pro-Moscow militia member were killed in a shootout at a Ukrainian military facility in Crimea's capital, Simferopol.
Thousands of Russian soldiers have overtaken Crimea in recent weeks. The majority-Russian region voted in a referendum Sunday to break away from Ukraine and join Russia.
Ukraine's defense minister, Andriy Parubiy, said Wednesday that Ukraine will ask the United Nations to declare Crimea a demilitarized zone and facilitate the withdrawal of both Russian and Ukrainian military forces from the peninsula.
Ukraine's deputy prime minister and defense minister traveled Wednesday from Kyiv to Crimea in an attempt to defuse tensions, but were denied entry into the region. International observers have been repeatedly turned away from the peninsula.
United Nations Secretary-General Ban Ki-moon was also traveling to the region Wednesday. He is to meet in Moscow with Russian President Vladimir Putin and Russian Foreign Minister Sergei Lavrov, and Thursday is to meet in Kyiv with Ukraine's acting president and prime minister.
President Putin's moves to annex Crimea have angered the United States and European Union, which have declared the referendum illegal and imposed sanctions on Russia in response.
U.S. Vice President Joe Biden told reporters in Vilnius, Lithuania on Wednesday that Russia will face "increasing political and economic isolation" as long as it continues on what he called "its dark path."
Biden is seeking to reassure eastern European countries - including the Baltic states Lithuania, Latvia and Estonia, all of which are NATO members, of U.S. support for its allies in the region.
NATO chief Anders Fogh Rasmussen said during a speech in Washington Wednesday that the crisis in Crimea is "the most serious security crisis since the end of the Cold War."
Mr. Putin told the Russian parliament Tuesday the Crimean peninsula has always been an "inalienable" part of Russia.
Also Tuesday, U.S. Secretary of State John Kerry said he was surprised and disappointed by what he called Mr. Putin's interpretation of the facts.
Kerry said Russia is on the wrong side of history and that when a region secedes from a country, it does it under the constitution - not at the butt of a gun.
Russia has shown no signs of backing down. Foreign Minister Sergei Lavrov told Kerry by phone Tuesday that U.S. sanctions on Russia are unacceptable and will have consequences.