Ukraine is drawing up plans to withdraw its soldiers and their families from Crimea, Kiev's security chief says.
Andriy Parubiy said they wanted to move them "quickly and efficiently" to mainland Ukraine.
Earlier, pro-Russian forces seized two naval bases - including Ukraine navy's HQ - in Crimea. Kiev says its navy chief has been detained.
It comes a day after Crimean leaders signed a treaty with Moscow absorbing the peninsula into Russia.
A referendum in Crimea on Sunday, approving its split from Ukraine, came nearly a month after Kiev's pro-Russian President Viktor Yanukovych was replaced by Western-leaning interim authorities.
Nato Secretary General Anders Fogh Rasmussen has called the crisis in Crimea "the gravest threat to European security and stability since the end of the Cold War".
There were charged exchanges in a session of the UN Security Council, during which US envoy Samantha Power said her Russian counterpart Vitaly Churkin "showed more imagination than Tolstoy or Chekhov".
"Russia it seems has re-written its borders but it cannot rewrite the facts," said Ms Power, who was then accused by Mr Churkin of dropping "to the level of the tabloid press".
Mr Parubiy, in a news conference, set out more details on Kiev's position in light of the events in Crimea.
He said arrangements were now being set up to introduce visas for Russian nationals travelling to Ukraine.
And he said Kiev was seeking UN support to "proclaim Crimea a demilitarised zone", which would involve the withdrawal of Russian troops and the "relocation of Ukrainian troops to continental Ukraine as well as facilitate evacuation of all the civilian population who are unwilling to remain on the occupied territory".
Ukraine is also leaving the Moscow-led Commonwealth of Independent States (CIS) alliance, and is preparing for military exercises with the US and the UK, Mr Parubiy added.
With reference to plans to withdraw troops and their families, Ukraine's interim Foreign Minister Andriy Deshchytsya told the BBC that they would not be forced to leave if they did not want to.
But he said: "The situation is unpredictable and uncontrolled sometimes, so that's why there is a danger also for the civilians".
Meanwhile, a deadline of 21:00 local time (19:00 GMT) set by Ukraine's interim President Olexander Turchynov for the release of navy chief Serhiy Hayduk has passed.
Shortly afterwards, Russian Defence Minister Sergei Shoigu called on the Crimean authorities to release him.
A defence ministry statement said Commander Hayduk had been obliged to carry out orders in accordance with Ukrainian military regulations.
Mr Turchynov earlier said that unless Serhiy Hayduk and "all the other hostages - both military and civilian ones - were released, the authorities would carry out an adequate response... of a technical and technological nature".
It is not clear exactly what he means, but it could involve the electricity or water that Ukraine supplies to Crimea, the BBC's David Stern in Kiev suggests.
Kiev said Mr Hayduk was detained soon after Ukraine's naval headquarters was stormed by some 200 pro-Russian activists, some armed, in Sevastopol - the port city which is also home to Russia's Black Sea fleet.
They were filmed going through offices, removing Ukrainian insignia and replacing Ukraine's flag with the Russian tricolour.
There were cheers from the crowd when Russia's Black Sea Fleet commander Aleksandr Vitko arrived and entered the building.
A handful of Ukrainian servicemen have refused to surrender. One told the BBC's Mark Lowen that they had been told to stay overnight to protect the equipment but fully expected to be told by Kiev in the morning to withdraw.
Ukraine's navy base in Novo-Ozyorne in west Crimea was also infiltrated after a tractor was used to ram the front gates. Some 50 Ukrainian servicemen were seen filing out of the base.
Ukrainian Defence Minister Ihor Tenyukh and First Deputy Prime Minister Vitaly Yarema reportedly tried to enter Crimea to defuse tensions but were prevented from doing so.
Earlier on Wednesday, Russia's constitutional court approved the treaty absorbing Crimea into the Russian Federation. The treaty now only needs ratifying by parliament which correspondents say it is certain to do.
In an emotionally charged speech on Tuesday, President Vladimir Putin said Crimea had "always been and remains an inseparable part of Russia".
Meanwhile, shocking footage has emerged of MPs from Ukraine's far-right Svoboda party roughing up Oleksandr Panteleymonov, the acting chief executive of the state broadcaster, over his decision to broadcast the treaty ceremony in the Kremlin.
The crisis in Crimea is expected to dominate a meeting of European Union leaders who meet in Brussels on Thursday.
UK Prime Minister David Cameron said the EU must send "a very clear warning" to Russia, raising the possibility of further sanctions against Moscow. He also said the G8 group should discuss whether to expel Russia "if further steps are taken".
Moscow said any expansion of sanctions was "unacceptable and will not remain without consequences".
UN Secretary General Ban Ki-moon is heading to the region. He will meet Mr Putin in Moscow on Thursday and Ukraine's interim leaders in Kiev on Friday.
Pro-Russian forces effectively took over Crimea - with its predominantly ethnic Russian population - after Mr Yanukovych fled Ukraine on 22 February following protests in which more than 80 people were killed.
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