Ukraine said on Thursday it would regard any movements by Russian military in Crimea outside the Russian Black Sea fleet's base in Sevastopol as an act of aggression.
Ukraine's Foreign Ministry summoned Russia's acting envoy in Kiev on Thursday and called for immediate consultations with Moscow following the seizure of the regional government and parliament buildings in Crimea.
It said in a written statement that the envoy, Andrei Vorobyov, had been handed a note requesting that Russian military based in the Crimean port of Sevastopol stay on base.
Russia recalled its ambassador from Kiev for consultations in Moscow after parliament stripped President Viktor Yanukovich of his powers on Saturday. The armed men in Crimea have raised the Russian flag over the regional government and parliament buildings, indicating they may be ethnic Russian separatists.
Acting President Oleksander Turchinov issued the warning in the national parliament after armed men seized the regional government and parliament buildings in Crimea, where some ethnic Russians want the region to join the Russian Federation.
Fighter jets along Russia's western borders have been put on combat alert, the Russian Defense Ministry was quoted as saying on Thursday by Interfax news agency.
"Constant air patrols are being carried out by fighter jets in the border regions," Interfax quoted a ministry statement as saying. "From the moment they received the signal to be on high alert, the air force in the western military region left for the ... air bases."
Poland's Foreign Minister Radosaw Sikorski said that the seizure of the government buildings in Crimea is a "drastic step", risking a regional conflict. "This is a drastic step and I'm warning those who did this and those who allowed them to do this, because this is how regional conflicts begin. This is a very dangerous game," Sikorski told a news conference.
Ukraine put its police on high alert after dozens of armed pro-Russia men stormed and seized local government buildings in Ukraine's Crimea region early Thursday and raised a Russian flag over a barricade.
The renewed tension in this strategic peninsula that houses Russia's Black Sea fleet comes as lawmakers in Kiev were expected to approve the new government in the wake of the president fleeing the capital after months of protests over his decision to scuttle an agreement with the European Union in favor of stronger ties with Russia.
Russia has questioned the legitimacy of the new Ukrainian authorities after pro-Russian President Viktor Yanukovych fled last week, and it has accused them of failing to control radicals who threaten the Russia-speaking population in Ukraine's east and south, which includes the Crimean Peninsula.
The men occupying the local parliament building did not immediately voice any demands but threw a flash grenade in response to a journalist's questions. They wore black and orange ribbons, a Russian symbol of the victory in World War II, and put up a sign saying "Crimea is Russia."
Maxim, a pro-Russian activist who refused to give his last name, said he and other activists who had camped out overnight outside the local parliament in Crimea's regional capital, Simferopol, when heavily armed men wearing flak jackets, and holding rocket-propelled grenade launchers and sniper rifles took over the building.
"Our activists were sitting there all night calmly, building the barricades," he said. "At 5 o'clock unknown men turned up and went to the building. They got into the courtyard and put everyone on the ground.
"They were asking who we were. When we said we stand for the Russian language and Russia, they said: don't be afraid, we're with you. Then they began to storm the building bringing down the doors," he said.
"They didn't look like volunteers or amateurs, they were professionals. This was clearly a well-organized operation. They did not allow anyone to come near. They seized the building, drove out the police, there were about six police officers inside," he said.
"Who are they? Nobody knows. It's about 50-60 people, fully armed," he said.
The events unfolding in Crimea highlighted the divided allegiances between Russia and the West that have deepened amid the political turmoil that has gripped Ukraine in recent months.
Russian President Vladimir Putin on Wednesday put the military on alert for massive exercises involving most of the military units in western Russia, and the military announced measures to tighten security at the headquarters of Russia's Black Sea Fleet on the peninsula.
The maneuvers will involve some 150,000 troops, 880 tanks, 90 aircraft and 80 navy ships, and are intended to "check the troops' readiness for action in crisis situations that threaten the nation's military security," Defense Minister Sergei Shoigu said in remarks carried by Russian news agencies.
The move prompted a sharp rebuke from U.S. Secretary of State John Kerry, who warned Russia against any military intervention in Ukraine.
Russia denied the military maneuvers were connected to the situation in Ukraine, but the massive show of force appeared intended to show both the new Ukrainian authorities and the West that the Kremlin was ready to use all means to protect its interests.
Ukraine's acting interior minister Arsen Avakov said on his Facebook page on Thursday that areas around the occupied buildings were being sealed off by police.
"Measures have been taken to counter extremist actions and not allow the situation to escalate into an armed confrontation in the center of the city," he said.
NATO Secretary-General Anders Fogh Rasmussen said on Thursday he was concerned about developments in Ukraine's Crimea and urged Russia not to do anything that would "escalate tension or create misunderstanding."
"I'm concerned about developments in Crimea. I urge Russia not to take any action that can escalate tension or create misunderstanding," Rasmussen said on Twitter.
Phone calls to the Crimean legislature rang unanswered, and its website was down. Refat Chubarov, a local leader of the Tatar community that support the new authorities in Kiev, wrote on his Facebook page early Thursday that the two buildings were taken overnight by uniformed men.
The events in Crimea came as Ukraine's Parliament was to form a new government after three months of street protests and violent clashes resulted in Yanukovych fleeing the capital.
Protesters took to the streets after his decision in November to reject an agreement that would strengthen ties with the EU and instead seek closer cooperation with Moscow. The protests expanded to include grievances against corruption and human rights abuses.
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