Speaking at Berlin's historic Brandenburg Gate, a powerful symbol of the Cold War, he said it was possible to ensure American security and a strong deterrent and also reduce nuclear weapons.
He said he intends to seek negotiated cuts with Russia to move beyond a Cold War nuclear posture.
Mr Obama promised to work with Nato allies to forge a new international framework for peaceful nuclear power. He said the US will hold a summit in 2016 on securing nuclear materials around the world.
He also called for a treaty to end production of fissile material.
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