In what is considered an unprecedented effort to rally global action on securing vulnerable nuclear materials, President Barack Obama will host leaders from 46 countries for a two-day nuclear security summit at the Walter E. Washington Convention Center in the nation's capital, starting Monday, April 12, 2010. The summit will focus on how to effectively safeguard nuclear weapons and technology, from of the desperate reach of terrorists.

As the summit begins, the United States is negotiating with the four other permanent members of the United Nations Security Council on firmer 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 ... would be the possibility of a terrorist organization obtaining a nuclear weapon," Obama said Sunday, appearing with South African President Jacob Zuma in Washington.

"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. We know that organizations like al Qaeda are in the process of trying to secure nuclear weapons or other weapons of mass destruction and would have no compunction at using them." He added.

Obama signed a new treaty with Russia last week to reduce the nuclear stockpiles of both nations, and his administration issued a revised U.S. nuclear arms strategy intended to reinforce the nation's nuclear deterrent while isolating terrorists states that fail to comply with international regulations.

It all fits together as an ambitious effort to mobilize a unified global effort against nuclear proliferation, Secretary of State Hillary Clinton and Defense Secretary Robert Gates said in interviews broadcast Sunday. She added that major concerns are older nuclear weapons and materials in countries like Russia and other states of the former Soviet Union are not properly safeguarded.

The summit's goal is to get an agreement and working plan on how each of the participating countries will control nuclear materials or otherwise prevent their spread, Clinton said on a CBS program "Face the Nation." White House press secretary Robert Gibbs announced last week that nations participating in the summit would be Algeria, Argentina, Armenia, Australia, Belgium, Brazil, Canada, Chile, China, the Czech Republic, Egypt, Finland, France, Georgia, Germany, India, Indonesia, Israel, Italy, Japan, Jordan, Kazakhstan, Malaysia, Mexico, Morocco, Netherlands, New Zealand, Nigeria, Norway, Pakistan, Philippines, Poland, the Republic of Korea, the Russian Federation, Saudi Arabia, Singapore, Switzerland, South Africa, Spain, Sweden, Thailand, Turkey, United Arab Emirates, the United Kingdom, Ukraine and Vietnam, the United Nations, the European Union and the International Atomic Energy Agency will be represented at the summit.

Obama also is holding bilateral meetings with some of the visiting heads of state, which started Sunday afternoon with Indian Prime Minister Manmohan Singh, followed by President Nursultan Nazarbayev of Kazakhstan, South African President Jacob Zuma, Prime Minister Yusuf Raza Gilani of Pakistan and acting President Goodluck Jonathan of Nigeria.

Meanwhile, Obama's nuclear strategy has been criticized by Republicans as too weak in the face of threats and defiance by Iran and North Korea. "While the treaty [with Russia] may be in the right direction and the nuclear summit that's coming to town may be an impressive group of people, the nuclear posture statement that the president put out is troublesome to me," Republican Sen. Lamar Alexander of Tennessee said on the "Fox News Sunday" program.

On Sunday, Iran's state-run Islamic Republic News Agency (IRNA) reported that Supreme Leader Ayatollah Ali Khamenei said the Obama administration "poses a threat to international peace" and is "wicked and untrustworthy." Based on the news agency's report, Khamenei said Obama had recently threatened Iran with nuclear weapons and called for Iran's military to prepare itself. Khamenei apparently was referring to last week's announced shift in U.S. nuclear strategy, in which the administration said it would swear off developing new generations of nuclear weapons and would not use its existing arsenal to attack non-nuclear states that are in compliance with nonproliferation agreements.

But Clinton defended the Obama administration's policy of seeking a unified international response to Iran's nuclear ambitions, including stronger U.N. sanctions.