escalation in the ukraine conflict. now u.s. officials are hunting for signs whether he is seriously considering using nuclear force. nina dos santos with the story. >> reporter: as russia's tanks rolled into ukraine, vladimir putin made a threat not heard since the height of the world war. >> translator: russia's response will be immediate and will lead you to such consequences never experienced in your history. >> reporter: then, days later, he raised the alert level of the world's largest nuclear arsenal. >> translator: everyone knows that a third world war can only be nuclear. >> reporter: only nine countries have nuclear weapons. hopefully never to be needed in battle. according to the armed control association, russia has the largest number of war heads at just over 6,000. while the u.s. isn't far behind, no other country, not even israel or north korea, has anywhere near this type of capability.