Thank you for your great answers! It's more clear but more questions arise. What do ATC do with civil aircraft in the near vicinity? If they fly super sonic and/or change altitude, direction rapidly? Who will be responsible if they collide with civil aircraft in IMC conditions? Do Russians use electronic counter measures to deceive or evade radar?
I don't remember reading about this in air law but if I fly private aircraft in international airspace can I also turn off transponder and do not talk to ATC? Will I be fined for this?