Obama hosts two-day summit on nuclear security

President Obama hosts leaders from 46 countries for a two-day nuclear security summit starting Monday that will focus on how to better safeguard weapons materials, both old and new, to keep them out of the hands of terrorists.

The gathering at the Walter E. Washington Convention Center in the nation's capital is considered an unprecedented effort to rally global action on securing vulnerable nuclear materials.

It also is the centerpiece of a major Obama objective aimed at preventing the spread of nuclear weapons and technology.

As the summit begins, the United States is negotiating with the four other permanent members of the United Nations Security Council on tougher sanctions against Iran over its nuclear ambitions.

"The central focus of this nuclear summit is the fact that the single biggest threat to U.S. security, both short term, medium term and long term, would be the possibility of a terrorist organization obtaining a nuclear weapon," Obama said Sunday, appearing with South African President Jacob Zuma in Washington.