The future of mankind is at stake and all nations must strive to rid the world of nuclear arms, the US President has said.
As North Korea launched a long-range rocket over Japan, Barack Obama dismissed those who warned the proliferation of nuclear weapons could not be checked.
"This fatalism is a deadly adversary," he told a crowd of 20,000 in Prague. "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."
President Obama said his goal of "a world without nuclear weapons" won't be reached soon, "perhaps not in my lifetime," but he said his country, with one of the world's largest arsenals and the only nation to have used an atomic bomb, had a "moral responsibility" to start taking steps now.
Addressing his American audience, President Obama said surrendering nuclear weapons must be a global "all for one" endeavour.
"Make no mistake: As long as these weapons exist, we will maintain a safe, secure and effective arsenal to deter any adversary, and guarantee that defense to our allies."
He also gave his most unequivocal pledge yet to proceed with a missile defence system in Europe.
The shield will be based in the Czech Republic and Poland, on Russia's doorstep.
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