Russia's Defence Ministry says fighter jets along the country's western borders have been put on combat alert.
It follows reports armed men have seized a regional government building and the parliament in the Crimea region of Ukraine.
The country's acting president Olexander Turchynov has appealed for calm, saying he's given his security staff permission to take 'all measures' to protect citizens.
Meanwhile Ukraine's deposed President Viktor Yanukovych has been seen in Russia's capital Moscow.
He's believed to have asked the authorities there to guarantee his safety - and is thought to be staying in a property owned by the Kremlin.
