Fighter jets along Russia's western borders have reportedly been placed on combat alert amid continuing unrest in the Crimea region of Ukraine.
The exercise came a day after US Secretary of State John Kerry warned Russian President Vladimir Putin to remember that the escalating crisis "is not Rocky IV".
Meanwhile, Ukrainian fugitive president Viktor Yanukovych - who is seeking refuge in a luxury health resort outside Moscow, Russian news agency RBC has claimed - has allegedly asked for Russia's protection from "extremists".
He also said he still considers himself to be the legitimate leader, according to a statement published by three Russian news agencies. An unnamed Russian official said his request was "satisfied in the territory of Russia."
A spokesperson for Mr Putin said they were not aware of Mr Yanukovych's reported arrival in Moscow.
Ukraine’s acting president Oleksandr Turchynov issued a warning to Russia that any movement of its troops from their naval base in the southern Crimea region “will be considered military aggression."
In pictures: Crisis in Ukraine
In pictures: Crisis in Ukraine
1/11 Demonstrations in Crimea
Crimean Tatars drag away a police officer in front of a local government building in Simferopol
2/11 Demonstrations in Crimea
Pro-Russian activists pray outside the Crimean Parliament building in Simferopol
3/11 Demonstrations in Crimea
Crimean Tatars hold flags during rallies near the Crimean parliament building in Simferopol
4/11 Demonstrations in Kiev
Young girls chant the song 'Glory to Ukraine' at Independence Square in central Kiev
5/11 Demonstrations in Crimea
A man receives medical treatment after he was injured in clashes during rallies near the Crimean parliament building
6/11 Demonstrations in Crimea
Pro-Ukrainian activists hold placards reading "Crimea +Ukraine is love" during a rally in front of the Crimean parliament in Semfiropol
7/11 Demonstrations in Kiev
Protestors capture a military armoured vehicle in central Kiev
8/11 Demonstrations in Kiev
An old man stands with an Ukrainian flag on Kiev's Independence Square
9/11 Demonstrations in Kiev
A man lays flowers at one of the barricades heading to Kiev's Independence Square
10/11 Demonstrations in Kiev
Flowers cover the ground and barricades where protesters were killed in a recent clash with riot police in Kiev's Independence Square
11/11 Demonstrations in Kiev
A self-defense unit patrolling the city centre in Kiev, Ukraine
Mr Turchynov also declared the takeover of government buildings in Crimea as a "crime against the government of Ukraine. I have given orders to the military to use all methods necessary to protect the citizens, punish the criminals, and to free the buildings," he said.
The Russian Interfax news agency quoted the Defence Ministry as saying:"Constant air patrols are being carried out by fighter jets in the border regions. 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."
Mr Turchynov, who is also the head of Ukraine's armed forces, appealed for Moscow to adhere to the rules of an agreement which allows Russia's Black Sea fleet to be based in Sevastopol until 2042.
"I am appealing to the military leadership of the Russian Black Sea fleet ... 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," he declared.
Kerry made the comments in an apparent bid to distance the US from a Cold War-style confrontation with Russia. He said Russia must be “very careful” in any decisions it makes surrounding Ukraine, and that a military intervention in Ukraine would be a “ huge, grave mistake”.
“This is not Rocky IV,” he said, referring to the 1990 film depicting a battle between the East and West, in which Rocky Balboa fights Soviet Union boxer Ivan Drago.
“Believe me. We don’t see it that way", he told US TV Network NBC News.
“I think Russia needs to be very careful in the judgments that it makes going forward here,” he added. “We are not looking for confrontation. But we are making it clear that every country should respect the territorial integrity here, the sovereignty of Ukraine.
“Russia has said it would do that and we think it’s important that Russia keeps its word.”
Mr Kerry’s comments came after Mr Putin ordered the urgent drill test combat readiness of its military forces amid tensions with the West over Ukraine.
The exercise, involving 150,000 of precisely those forces which are likely to be used for any intervention, and their reserves, came amid growing claims that the country faces the threat of breaking up, as protests and violent sectarian clashes continue.
Russia said earlier this morning it would "uncompromisingly" defend the rights of its compatriots in Ukraine, while expressing concerns over "large-scale human rights violations", attacks and vandalism in the former Soviet republic.
Additional reporting by agenciesReuse content