Ukraine has proposed that Crimea, scene of a Russian-backed armed takeover, be declared a demilitarised zone by the United Nations with a pull-out by both Russian and Ukrainian forces.
"The Ukrainian government will immediately appeal to the United Nations to recognise Crimea as a demilitarised zone and take necessary measures for Russian forces to leave Crimea and prepare conditions for redeployment of Ukrainian forces," security chief Andriy Parubiy said on Wednesday.
Earlier, several men in civilian clothing, later joined by military-looking personnel believed to be Russian, had seized the Ukrainian naval headquarters in the Crimean port of Sevastopol.
Parubiy told journalists that in the face of the growing likelihood of military confrontation with Russia, the Ukrainian foreign ministry had been given the task of introducing visas for Russians visiting the former Soviet republic.
Referring to his demilitarisation proposal, Parubiy, secretary of the National Security and Defence Council, said he hoped the UN would support the idea.
"This formula is the best for de-escalation ... We hope that common sense will prevail," he said.
A naval spokesman on Wednesday said there had been no violence during the takeover of the base and that he believed the men belonged to so-called "self-defence" units, mainly made up of volunteers who have supported Crimea's transition from Ukrainian to Russian control, the Reuters news agency reported.
Dozens of despondent Ukrainian soldiers, one of them in tears, filed out of the base.
"We have been temporarily disbanded," a Ukrainian lieutenant who identified himself only as Vlad told the AFP news agency.
"I was born here and I grew up here and I have been serving for 20 years," he said as a Russian flag went up over the base without a single shot being fired in its defence. Where am I going to go?"
Sergiy Gayduk, the Ukraine navy commander appointed after his predecessor switched allegiance in favour of Crimea's pro-Kremlin authorities at the start of the month, was detained.
Three Russian flags were later seen flying at one of the entrances to the base.
Ukraine's acting president on Wednesday said Crimea's separatist leaders had three hours to release Gayduk or face "an adequate response".
Oleksandr Turchynov said in a statement that "unless Admiral Gayduk and all the other hostages - both military and civilians ones - are released, the authorities will carry out an adequate response ... of a technical and technological nature."
As tensions continued to rise, the AP news agency reported that Ukrainian Defence Minister Ihor Tenyukh had been denied entry to Crimea.
Earlier, at a cabinet meeting in the capital Kiev, acting Ukraine Prime Minister Arseniy Yatsenyuk had asked his first vice PM Vitaliy Yarema and Tenyukh to fly to Crimea immediately to de-escalate the crisis and try to prevent the conflict from turning into a military one. The officials immediately left for the airport.
Crimean Prime Minister Sergei Aksyonov responded on Wednesday by saying the officials would not be allowed to enter, Russian news agency Interfax reported.
"They are not wanted in Crimea. Nobody will let them into Crimea, they will be sent back," Interfax quoted Aksyonov as saying.
The Crimean authorities do not recognise the government in Ukraine, while the parliament in Kiev does not accept Crimea's referendum, held last Sunday, to join Russia, insisting it is legal.
Al Jazeera's Jennifer Glasse, reporting from inside the naval headquarters, said the group had decided to make its move after Ukrainian troops were told they could fire their weapons in self-defence.
"It has been a very tense situation for several weeks," she said, adding that most of the weapons on the base were kept out of reach to avoid incidents.
"The men who came in said they made the decision because the Ukrainians were authorised to shoot in self defence. It doesn't mean they are ordered to shoot, but that they are authorised to use their weapons.
"This morning not a single shot was fired."
Ukrainian authorities issued the self-defence order following the death of a Ukrainian serviceman on Tuesday.