"Terrorist networks such as al Qaeda have tried to acquire the material for a nuclear weapon, and if they ever succeeded, they would surely use it," he told the opening session, which convened under tight security at the Washington Convention Center. "Were they to do so, it would be a catastrophe for the world, causing extraordinary loss of life and striking a major blow to global peace and stability."

Lurking in the background at the nuclear security summit was a problem some see as equally worrisome: Iran's purported pursuit of a nuclear weapon. Iran, which was not invited to the conference, denies it intends to build an atomic bomb, and despite widespread concern about Iranian intentions, Mr. Obama is having difficulty getting agreement on a new set of U.N. sanctions against the country.

Mr. Obama organized the nuclear summit to win agreement on a plan for securing all vulnerable nuclear materials worldwide within four years. The summiteers were expected to announce how they think that can be done, with plans to review progress at a follow-up conference in South Korea in 2012.

President Lee Myung-bak told reporters that North Korean leader Kim Jong-il will not get an invitation until the North gives up its efforts to build a nuclear arsenal.

North Korea's efforts -- and its withdrawal from the Nuclear Non-Proliferation Treaty that sets the rules of the road for nuclear technology -- kept it out of the Washington summit. Syria, which is suspected by the U.S. and others of harboring nuclear weapons ambitions, also was not invited.

As an example of the collective action called for by Mr. Obama, officials of the U.S., Canada and Mexico announced an agreement to work together, along with the U.N.'s nuclear watchdog agency, to convert the fuel in Mexico's research reactor from highly enriched uranium to a lower-enriched fuel that would be much harder to use in the manufacturing of a nuclear weapon.

Mexico further agreed that once the fuel is converted, it will get rid of all its highly enriched uranium. That follows Ukraine's announcement on Monday that it, too, will ship all its highly enriched uranium to protected storage outside its borders -- possibly to Russia or the U.S.

U.S. officials touted their completion of a long-delayed agreement with Russia on disposing of tons of plutonium from Cold War-era weapons. Both countries will complete and operate facilities to dispose of at least 34 tons of plutonium by using it as fuel in civilian power reactors to produce electricity, although it will not start until 2018; monitors and inspectors will ensure against cheating.

The State Department said the 34 tons of plutonium represents enough for about 17,000 nuclear weapons. The deal was being signed Tuesday by Secretary of State Hillary Rodham Clinton and Russian Foreign Minister Sergey Lavrov.

Signaling a growing commitment in Europe to addressing the dangers of nuclear proliferation and potential nuclear conflict, a group of 40 former European politicians and military officers issued a statement endorsing the goal of the Washington conference.

They also said nuclear dangers cannot be contained by addressing the terrorist threat alone. The nuclear powers need to disarm faster, they said, and countries that have not yet signed the Comprehensive Test Ban Treaty -- including the United States -- must do so.

"This is not just a concern for those fearing a nuclear terrorist attack," the Europeans' statement said. "Any major nuclear security incident anywhere is likely to derail the civil nuclear renaissance everywhere."