Russia has deployed its advanced S-400 air defense missile system to Crimea. The announcement comes after President Vladimir Putin promised to take measures against what he called "terrorist attacks" by Ukrainian military intelligence in northern Crimea.

Prime Minister Dmitry Medvedev says Russia may even cut diplomatic ties with Ukraine if there are no other options available. Kiev has denied Moscow's allegations.

On Friday, Ukraine's security service initiated the highest terrorist threat level in Crimea.  It includes the eastern regions of Donetsk and Lugansk, where anti-Kiev rebels have been fighting government troops for more than two years.