Moscow said about 150,000 troops would take part in exercises in the Western military district that borders Ukraine, although it denied that the previously unannounced war games were linked to the Ukrainian crisis.
Moscow denounced what it described as the rise of "nationalist and neo-fascist sentiment" in Ukraine's mainly Ukrainian-speaking west, where it said Russian speakers were being deprived of rights.
On Thursday, the Russian Defense Ministry said fighter jets along the country's border with Ukraine had been put on combat alert, according to the 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."
Secretary of State John Kerry said late Wednesday it would be a grave mistake for Russia to send troops into neighboring Ukraine.In Kiev, opposition leaders who took charge after Yanukovich fled were working on forming a new government to chart a path forward for the country and its ailing economy. The parliament approved Arseniy Yatsenyuk Thursday as the nation's new prime minister.
Opposition leaders chose Independence Square -- the center of the revolution -- to introduce the country's new government Wednesday. The introductions prompted booing from some opposition supporters in the square.
Parliament must ratify the new ministers Thursday, but CBS News correspondent Clarissa Ward reported that for some in the crowd, there were too many familiar faces from the Yanukovich days on stage.
Russia has repeatedly expressed concern for the safety of Russian citizens in Ukraine, using language similar to statements that preceded its invasion of Georgia in 2008.
Since Yanukovich's downfall on Saturday, all eyes have been on Putin, who ordered the invasion of Georgia to protect two self-declared independent regions with many ethnic Russians and others holding Russian passports. Moscow recognized the regions as independent states, using armed force effectively to seize control of territory from its neighbor.
Any military action in Ukraine, a country of 46 million people that has close ties with European powers and the United States, would be far more serious -- the closest the West and Russia have come to outright confrontation since the Cold War.


