A former U.S. defense secretary is on a mission to warn of a "real and growing danger" of nuclear doom, troubled by the risks of catastrophe from the very weapons he helped develop.

Atop William J. Perry's list: a nuclear terror attack in a major U.S. city or a shooting war with Russia that, through miscalculation, turns nuclear. A terrorist attack using a nuclear bomb or improvised nuclear device could happen "any time now - next year or the year after," he said in an interview with reporters this month.

Perry, 88, chooses his words with the precision of a mathematician, which he was before entering the defense world in the mid-1950s. He played a central role in developing and modernizing nuclear forces throughout the Cold War - first as a technology whiz-kid and later a three-time senior Pentagon executive. During the 1962 Cuban missile crisis Perry was secretly summoned to Washington to analyze intelligence on Soviet weapons in Cuba.