I am appealing to the military leadership of the Russian Black Sea fleet, said Olexander Turchinov, acting president since the removal of Viktor Yanukovich last week. Any military movements, the more so if they are with weapons, beyond the boundaries of this territory (the base) will be seen by us as military aggression.
Ukraines foreign ministry also summoned Russias acting envoy in Kiev for immediate consultations.
There were mixed signals from Moscow, which put fighter jets along its western borders on combat alert, but earlier said it would take part in discussions on an International Monetary Fund (IMF) financial package for Ukraine. Ukraine has said it needs $35 billion over the next two years to stave off bankruptcy.
The fear of military escalation prompted expressions of concern from the West, with NATO secretary-general Anders Fogh Rasmussen urging Russia not to do anything that would escalate tension or create misunderstanding.
Polish foreign minister Radoslaw Sikorski called the seizure of government buildings in the Crimea a very dangerous game. This is a drastic step, and Im warning those who did this and those who allowed them to do this, because this is how regional conflicts begin, he told a news conference.
It was not immediately known who was occupying the buildings in the regional capital Simferopol and they issued no demands, but witnesses said they spoke Russian and appeared to be ethnic Russian separatists.
Interfax news agency quoted a witness as saying there were about 60 people inside and they had many weapons. It said no one had been hurt when the buildings were seized in the early hours by Russian speakers in uniforms that did not carry identification markings.
We were building barricades in the night to protect parliament. Then this young Russian guy came up with a pistol ... we all lay down, some more ran up, there was some shooting and around 50 went in through the window, Leonid Khazanov, an ethnic Russian, told Reuters.
Theyre still there ... Then the police came, they seemed scared. I asked them (the armed men) what they wanted, and they said To make our own decisions, not to have Kiev telling us what to do, said Khazanov.
About 100 police were gathered in front of the parliament building, and a similar number of people carrying Russian flags later marched up to the building chanting Russia, Russia and holding a sign calling for a Crimean referendum.