Simferopol, Crimea (CNN) -- A day after Russia claimed Crimea as its own, there are signs the uneasy standoff between pro-Russian and Ukrainian forces could ignite into bloody conflict.
Almost 300 armed pro-Russian supporters took over the Ukrainian navy's headquarters in the Crimean port of Sevastopol on Wednesday, said Marina Kanalyuk, assistant to the commander of Ukraine's navy fleet.
"They are everywhere here, they surround us, they threaten us," she said, adding that she was sure that Russian security forces were involved.
Kanalyuk said the 70 or so Ukrainian naval officers at the headquarters had tried to stop the armed men from entering and were negotiating with them. She said that the armed men had replaced Ukrainian flags with Russian ones but that no shots had been fired.
A little later, Vladislav Seleznyov, Defense Ministry spokesman in Crimea, said a group of people had taken Ukrainian navy chief Sergey Gaiduk from the navy headquarters.
Russia's official ITAR-Tass news agency, citing the local Kryminform news agency, reported that Gaiduk had been passed to the Sevastopol prosecutor's office to be questioned about whether he'd passed on orders from Kiev for Ukrainian soldiers to use their weapons.
The incident comes a day after one member of the Ukrainian military was killed, another wounded and more captured when masked gunmen seized their base near the Crimean regional capital, Simferopol.
After that fatality -- the first Ukrainian military death since the Crimea crisis erupted about two weeks ago -- Ukraine's Defense Ministry authorized its forces to open fire in self-defense.
On the same day, Russian President Vladimir Putin announced the annexation of Crimea, after voters in the semi-autonomous territory approved a hastily called weekend referendum on separating from Ukraine.
U.N. chief to visit Russia, Ukraine
As diplomatic efforts to resolve the crisis continue, the United Nations said Secretary-General Ban Ki-moon would visit Russia and Ukraine this week "as part of his diplomatic efforts to encourage all parties to resolve the current crisis peacefully."
He will meet with Putin and Russian Foreign Minister Sergey Lavrov in Moscow on Thursday, the United Nations said, and with Ukrainian interim President Oleksandr Turchynov and Prime Minister Arseniy Yatsenyuk on Friday in Kiev.
U.S. Vice President Joe Biden, dispatched to reassure NATO allies in Eastern Europe and the Baltics, met Wednesday with Lithuanian President Dalia Grybauskaite and Latvian President Andris Berzins in Lithuania's capital, Vilnius.
He stressed that the United States was committed to the defense of its NATO allies and had stern words for Moscow's response to the political upheaval in Ukraine.
"Russia has chosen to respond with military aggression, a referendum rejected by virtually the entire world, illegal efforts to annex Crimea and now reports of armed attacks against Ukrainian military personnel and installations in Crimea," he said.
"I want to make it clear: We stand resolutely with our Baltic allies in support of the Ukrainian people and against Russian aggression."
On Tuesday, after talks in Poland, Biden warned that Russia faces "more than just sanctions" unless it abandons its "land grab" in Crimea.
Yatsenyuk warned Tuesday that the crisis was shifting "from political to the military form, and the blame is on the Russian military."
His Deputy Prime Minister Vitaly Yarema and acting Defense Minister Ihor Tenyukh are traveling from Kiev to Crimea on Wednesday "to prevent the escalation of conflict," said Sergiy Nahoryanski, head of the Ukrainian government's press center.
It's not clear whether they will be able to gain access to the region. International observers have been turned away from Crimea in recent days, and flights have also been limited.
The weekend's contested referendum has been condemned by the interim Ukrainian government in Kiev, the United States and the European Union.
But Russia remains steadfastly defiant.
Putin: Crimea has always been Russian
Russia's Constitutional Court unanimously ruled Wednesday that the agreement between the Russian Federation and Crimea on its accession to Russia was lawful, state news agency RIA Novosti reported.
The step clears the way for the country's lawmakers to vote on ratifying the accession agreement, as well as draft amendments to the Russian Constitution, it said.
The State Duma, or lower house, will hold a special session Thursday to ratify the treaty, ITAR-Tass reported.
Putin made it clear Tuesday, in a speech greeted by enthusiastic applause by Russian lawmakers, that Moscow has no intention of relinquishing its grip on Crimea, a Black Sea peninsula with historical ties to Russia.
"In our hearts, we know Crimea has always been an inalienable part of Russia," he said.
Of those who cast a ballot, 96.7% voted in favor of Crimea leaving Ukraine and joining Russia, according to the Crimean Electoral Commission. But the ballot questions offered no real option for any residents who wanted to remain part of Ukraine.
Putin said that Russia had to act because Ukraine's new government, backed by the United States and European powers, were prepared "to seize the state through terror and murders."
But Yatsenyuk called it "a robbery on an international scale," one that Kiev will never accept.
Members of the Russian parliament and the new Crimean legislature met Wednesday morning in Moscow to discuss the details of how the region will join Russia.
Crimean lawmakers have said they now see Ukrainian soldiers as an occupying force and have given them a certain amount of time to leave the peninsula.
The Ukrainian troops have been encircled by pro-Russian forces in their bases for days.
They have also come under pressure from pro-Russian crowds who gather outside and call on those still loyal to Kiev to surrender or defect to Russia's military. CNN teams on the ground have seen Russian forces also show up to reinforce that message.
Putin has denied that Russia's military has been used in Crimea, despite what has been stated by international observers and the government of Kiev. The pro-Russian forces seen in Crimea are Crimean "self-defense" forces, he said.
Putin said the 22,000 Russian troops in Crimea did not enter during the current crisis, but "were already there," in accordance with previous international negotiations.
Russian forces were allowed in Crimea under a treaty that allowed the Black Sea fleet to be based in the port of Sevastopol, but their movements within Crimea are supposed to be agreed upon with Kiev.
U.S. and EU officials imposed sanctions on more than two dozen Russian and Crimean officials Monday and urged Russia to avoid escalating the crisis, but Moscow has ignored those calls.
Tuesday's annexation brought a new round of condemnation from the West, with British Prime Minister David Cameron saying it sends "a chilling message across the continent of Europe."
Putin may be under international pressure over Russia's actions in Crimea, but public opinion is firmly behind him at home.
Cheering crowds celebrated the announcement that Crimea was now part of Russia at some 80 rallies across the country on Tuesday, from Vladivostok to Moscow's Red Square.
"'I love Putin a lot. I love Russia. We need to be together. We're for peace, we're for Putin, we're for Crimea, and we're for Sevastopol," said one woman in Moscow.
Posted by Greg Palmer