More than 40 world leaders are in Washington this week to participate in the Nuclear Security Summit.
Their goal is finding ways to secure the world's nuclear stockpile.
President Barack Obama says organizations like al-Qaida are in the process of trying to obtain nuclear weapons or other weapons of mass destruction. He calls that the "single biggest threat to U.S. security." He says if a terrorist organization acquired those weapons it "would have no compunction at using them."
The president says "If there was ever a detonation in New York City, or London, or Johannesburg, the ramifications economically, politically and from a security perspective would be devastating."
Although the two-day summit officially begins tomorrow, Obama has already been holding a series of bilateral meetings.
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