The gunmen raised Russian flags and banners at several government buildings in the capital of predominantly ethnic-Russian Crimea.
The move highlights the divided loyalties among residents of the autonomous republic of Crimea, where 60 percent of the population are ethnic Russians. Some of them have held demonstrations calling on Russia to protect them.
The Guardian reports that about 50 to 60 gunmen, armed with sniper rifles, grenade launchers, and flak jackets, stormed the buildings and raised Russian flags and a banner reading "Crimea is Russia." It is not yet clear who they are.
The area around the parliament and government headquarters has been sealed off by police, Ukraine’s acting Interior Minister Arsen Avakov said on his Facebook page. "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.
The overnight seizure of the buildings came as 150,000 Russian combat troops began ten days of military maneuvers – some of them close to Ukraine’s border with Russia – on President Vladimir Putin’s orders, reports The Christian Science Monitor.
"The Supreme Commander-in-Chief has set tasks to check the capability of troops to take actions to settle crisis situations that pose a threat to the military security of the country as well as terrorism-related, sanitary epidemiological and man-made emergencies," Defense Minister Sergei Shoigu told a meeting of military chiefs.
The saber rattling underscores Moscow’s anger at the overthrow of Ukraine’s pro-Russian president Viktor Yanukovych after two months of protests in the capital Kiev. It is also being seen as a warning that Moscow is ready to defend the interests of Russian speakers in eastern and southern Ukraine.
Alexander Konovalov, head of the Moscow-based Institute of Strategic Assessments, told USA Today that Putin's announcement is a signal to Ukraine, but it did not mean Russia had plans to invade its neighbor.
"There is a connection [to Ukraine], it's a psychological, demonstrative signal that Russia would protect its countrymen in the Crimea," he said. "I don't think these drills mean that Russia is going to invade.... It wants to show that it does have the strategic capability to deploy troops, that it's militarily prepared."
Russia has a strategic interest in Crimea: The Russian Black Sea fleet is based in the port of Sevastopol. Crimea was part of Russia until Soviet leader Nikita Khrushchev gave it to Ukraine – then part of the Soviet Union along with dominant partner Russia.
The maneuvers could possibly be a prelude to an invasion: Russia invaded neighboring Georgia in 2008 to support Russian-speaking separatists there. But the current exercises, due to run until March 7th, are being held in Russia’s Western and Central military districts, not in the Southern district which is closest to the Crimean peninsula.
Arseniy Yatsenyuk, Ukraine’s prime minister-designate, told the BBC that "We ask our Russian partners to provide to stick to their... obligations, we believe Russia would never intervene into Ukrainian domestic affairs and will refrain from any steps that would split Ukraine."
US Secretary of State John Kerry, meanwhile, issued a strong warning on Wednesday to President Putin against any military adventure. "I don't think there should be any doubt whatsoever that any kind of military intervention that would violate the sovereign territorial integrity of Ukraine would be a huge - a grave mistake," he told reporters in Washington.
Mr. Kerry is also trying to play down the idea of US-Russian rivalry over Ukraine, where an opposition-dominated, pro-European cabinet is due to be sworn in on Thursday after Mr. Yanukovych’s ouster and disappearance.
“We do not believe this should be an East-West, Russia-United States,” he told MSNBC. “This is not Rocky IV, believe me.”
The crisis in Ukraine exploded when Yanukovych made a last-minute decision in November not to sign an association and trade deal with the European Union, and later accepted a $15 billion loan from Moscow. That loan is now in question, and it is uncertain whether Western countries and international financial institutions are willing to provide the $35 billion in bailout loans that the incoming government says it needs over the next two years to avoid defaulting on its foreign debt.
"We are to undertake extremely unpopular steps, as the previous government and previous president were so corrupted that the country is in a desperate financial plight," Mr. Yatsenyuk told the BBC after the cabinet was announced.
"We are on the brink of a disaster and this is the government of political suiciders,” he added. "So welcome to hell.”