President Obama met individually with some visiting foreign leaders Monday before the start of a two-day summit of 47 nations that will focus on how to better safeguard nuclear 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.

Security concerns for the meeting locked down part of the city's downtown core, with streets closed and temporary fencing erected around the convention center. Parked military vehicles and city dump trucks blocked access at some points to all but official vehicles.

Obama met with five leaders on Sunday and held similar talks Monday, starting with King Abdullah II of Jordan. The two men chatted and laughed but said nothing to reporters before their meeting, which included top aides.

Other side meetings scheduled for Monday were with leaders from Malaysia, Ukraine, Armenia and President Hu Jintao of China. The bilateral talks were intended to bolster the summit that includes a working dinner Monday night and all-day plenary session chaired by Obama on Tuesday.

"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.

"This is something that could change the security landscape in this country and around the world for years to come. 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."

The summit also is the centerpiece of a major Obama objective aimed at preventing the spread of nuclear weapons and technology. As it 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.

In addition, 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 and rogue 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, U.S. officials say.

Of particular concern are older nuclear weapons and materials that aren't safeguarded as well as they should be, particularly in Russia and other states of the former Soviet Union, according to U.S. officials. A goal of the summit is to shift the world's attention to what used to be considered a U.S.-Soviet issue.

"Unfortunately, we have a situation in which there is a lot of loose nuclear material around the world," Obama said Sunday. "And so the central focus of this summit is getting the international community on a path in which we are locking down that nuclear material in a very specific time frame, with a specific work plan."

South Africa, he said, is a "moral leader" on the nuclear issue, since it began -- and then dismantled -- a nuclear program during the apartheid era.

The summit's goal is to get an agreement and work plan on how each of the participating countries will control nuclear materials or otherwise prevent their spread, Secretary of State Hillary Clinton said in an interview broadcast Sunday on the CBS program "Face the Nation."

Obama has made clear that he wants to prevent Iran from developing nuclear weapons and to halt North Korea's program. Neither nation was invited to the summit.

On Sunday, Iran's state-run Islamic Republic News Agency reported that Supreme Leader Ayatollah Ali Khamenei said the Obama administration "poses a threat to international peace" and is "wicked."

"The Americans did their best to imply that the Islamic Republic of Iran is not a trustworthy country on nuclear issues, but the fact is that those who possess such weapons and shamefully threaten others with such bombs are unreliable," Khamenei said.

In addition, Khamenei said Obama had recently threatened Iran with nuclear weapons and called for Iran's military to prepare itself, the news agency reported.

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 non-proliferation agreements.

One leader invited to the summit but not attending is Israeli Prime Minister Benjamin Netanyahu, who sent Deputy Prime Minister Dan Meridor instead. On Sunday, which was global Holocaust Remembrance Day, Netanyahu described the world's response to Iran's nuclear ambitions and threats against the Jewish state as inadequate.

"I call on all enlightened countries to rise up and forcefully and firmly condemn Iran's destructive intentions and to act with genuine determination to stop it from acquiring nuclear weapons," Netanyahu said.

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.

In addition, the United Nations, the European Union and the International Atomic Energy Agency will be represented, Gibbs said.

Obama held bilateral talks Sunday with South Africa's Zuma, Indian Prime Minister Manmohan Singh, President Nursultan Nazarbayev of Kazakhstan, Prime Minister Yusuf Raza Gilani of Pakistan and acting President Goodluck Jonathan of Nigeria.

At home, 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. "I mean, it takes away the ambiguity about our use of nuclear power. Ambiguity in foreign policy is sometimes very useful."

Clinton and Defense Secretary Robert Gates defended the administration policies as a comprehensive approach to nuclear arms issues in the 21st century.

Responding to Republican concerns that the new strategy softened the U.S. nuclear deterrent, Gates made clear it left Iran and North Korea vulnerable to a U.S. nuclear attack because they refuse to comply with non-proliferation agreements.

"All options are on the table" regarding Iran and North Korea, Gates said on the CBS program.

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

The policy, which broke with previous President George W. Bush's non-negotiation stance, has exposed Iran's intransigence, Clinton said on the NBC program "Meet the Press."

Now, with the U.N. negotiations on tougher sanctions, "the Iranians have been beating down the doors of every country in the world to try to avoid" a new sanctions resolution, Clinton said.

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