The Ukrainian government says it is planning to withdraw its troops from Crimea, where Russia has been taking formal control as its forces seize military installations.
National Security and Defense Council secretary Andriy Parubiy said Wednesday that Ukraine will seek UN support to turn Crimea into a demilitarized zone as it seeks to relocate armed forces to the mainland.
Ukraine's military, which is heavily outnumbered in Crimea, has come under increased pressure since the region was nominally incorporated into Russia on Tuesday.
Earlier Wednesday, masked Russian-speaking troops seized control over Ukraine's naval headquarters in the city of Sevastopol. A Ukrainian navy commander was also detained during that operation.
The seizure of the installation in Sevastopol -- home port of Russia's Black Sea Fleet -- came as Acting Ukrainian Defense Minister Ihor Tenyukh said his forces would not withdraw from Crimea despite being largely outnumbered and coming under increased pressure since the region was nominally incorporated into Russia on Tuesday.
Pro-Moscow Crimean authorities reportedly blocked Tenyukh and another Ukrainian official from traveling to the peninsula to try to defuse tensions. "They are not welcome in Crimea," Crimean Prime Minister Sergei Aksyonov was quoted saying by Interfax news agency. "They will not be allowed to enter Crimea. They will be sent back."
Tens of thousands of heavily armed Russian and pro-Russian troops reportedly are now patrolling in Crimea.
The Russian-speaking troops, who arrived on base after the storming, wore helmets, flak jackets and uniforms with no identifying insignia. By afternoon, they were in full control of the naval headquarters, a set of three-story boxy white concrete buildings with blue trim. It was not immediately clear how many, if any, Ukrainian servicemen remained on base.
The several hundred pro-Russian militiamen and Cossacks who captured the base reportedly met no resistance. It came a day after a confrontation between Ukrainian soldiers and pro-Russian militia left two dead.
Tenyukh said there were no reports of injuries in the raid.
The defense ministry said in its statement that Sevastopol base commander Rear Adm. Sergei Haiduk was detained by unknown people after the storming of the fleet headquarters.
Tenyukh said Ukrainian forces will not withdraw from Crimea despite the signed treaty between Russian President Vladimir Putin and Crimean leaders, Reuters reported. When asked by journalists outside a government meeting if Kiev would pull its forces out of the peninsula, Tenyuka replied: "No. We will stay."
Vice President Joe Biden, meanwhile, declared Wednesday the United States will respond to any aggression against its NATO allies.
Biden announced in Warsaw that in addition to new NATO exercises that will take place in Poland, the U.S. was considering rotating American forces to the Baltic region as a step toward ensuring the collective defense of NATO allies. Those forces could conduct ground and naval exercises, and engage in training missions.
Standing side by side with a pair of Baltic leaders in Vilnius, Lithuania, Biden said the U.S. was "absolutely committed" to defending its allies, adding that President Barack Obama plans to seek concrete commitments from NATO members to ensure the alliance can safeguard its collective security.
Biden said the U.S. stands resolutely with Baltic states in support of the Ukrainian people against Russian aggression.
"Russia cannot escape the fact that the world is changing and rejecting outright their behavior," Biden said, after meeting in Vilnius with Lituanian President Dalia Grybauskaite and Lativan President Andris Berzins.
Jubilant crowds in Moscow and other cities across Russia, meanwhile, hailed the Crimea annexation, while Ukraine's interim government called Putin a threat to the "civilized world and international security."
Russian news agencies on Wednesday cited Constitutional Court chairman Valery Zorkin as saying the treaty signed by Putin has been ruled valid, thus formally clearing another hurdle for Moscow to annex Crimea. The treaty now only requires ratification by the Russian parliament.
Thousands of troops under apparent Russian command took over Crimea two weeks before Sunday's hastily called referendum, seizing Ukrainian military bases, blockading others and pressuring Ukrainian soldiers to surrender their arms and leave.
Putin insisted Russia's military presence in Crimea was limited to those stationed under the terms of a treaty with Ukraine that allows Russia to have up to 25,000 troops at its Black Sea fleet base. Ukraine claims that Russia deployed further forces, however, and expressly went against its request for troops to remain confined within their barracks.
Russia's military reportedly started large-scale aviation exercises in the northwest Wednesday, officials said, in regions that do not border Ukraine.
Officials said the exercises involving jet fighters and bombers were being conducted close to Baltic ex-Soviet republics, Reuters reported.
Interfax reported that the drills involving more than 40 Sukhoi and MiG warplanes being held in regions including Leningrad, which borders NATO-member Estonia and Finland, are scheduled to end in late March.
The Associated Press and Reuters contributed to this report.