President Obama on Sunday launched an effort to rid the world of nuclear weapons, calling them "the most dangerous legacy of the Cold War" and saying the U.S. has a moral responsibility to lead as the only nation ever to have used one.

In a speech driven with new urgency by North Korea's rocket launch just hours earlier, Obama said the U.S. would "immediately and aggressively" seek ratification of a comprehensive ban on testing nuclear weapons. He said the U.S. would host a summit within the next year on reducing and eventually eliminating nuclear weapons, and he called for a global effort to secure nuclear material.

"Some argue that the spread of these weapons cannot be checked — that we are destined to live in a world where more nations and more people possess the ultimate tools of destruction," Obama said to a huge crowd in a square outside the Prague Castle gates.

"This fatalism is a deadly adversary," he said. "For if we believe that the spread of nuclear weapons is inevitable, then we are admitting to ourselves that the use of nuclear weapons is inevitable."

Obama at one point targeted his comments directly at North Korea, which launched a rocket late Saturday night in defiance of the international community. The president was awakened by an aide and told of the news, which occurred in the early morning hours in Prague.

"North Korea broke the rules once more by testing a rocket that could be used for a long range missile," Obama said. "This provocation underscores the need for action — not just this afternoon at the U.N. Security Council, but in our determination to prevent the spread of these weapons."

Addressing another longtime foe on the nuclear front, Obama said the U.S. will present Iran with "a clear choice," to join the community of nations by ceasing its nuclear and ballistic missile activity or face increased isolation and a nuclear arms race in the Middle East.

He said the U.S. also will proceed with development of a missile defense system in Europe as long as there is an Iranian threat of developing nuclear weapons. If that threat is removed, he said, "The driving force for missile defense in Europe will be removed. "

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