WASHINGTON — In full accord on a global threat, world leaders Tuesday endorsed President Barack Obama's call for securing all nuclear materials around the globe within four years to keep them out of the grasp of terrorists. They offered few specifics for achieving that goal, but Obama declared "the American people will be safer, and the world will be more secure" as a result.

Obama had called the 47-nation summit to focus world attention on the threat of nuclear terrorism, a peril he termed the greatest threat facing all nations and a "cruel irony of history" after mankind had survived the Cold War and decades of fear stoked by a U.S.-Soviet arms race.

A terrorist group in possession of plutonium no bigger than an apple could detonate a device capable of inflicting hundreds of thousands of casualties, he said.

"Terrorist networks such as al-Qaida 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."

The summit countries said they would cooperate more deeply with the United Nations and its watchdog arm, the International Atomic Energy Agency. They also said they would share information on nuclear detection and ways to prevent nuclear trafficking.

Several countries, including Ukraine, Mexico and Canada, declared their intention to give up highly enriched uranium as a step toward making it harder for terrorist groups or criminal gangs to steal or acquire a key ingredient in the making of atomic weapons. Russia and the U.S. signed a deal to dispose of tons of weapons-grade plutonium, although that won't start for eight years.

While the summit focused on the threat from terrorists, attention was given to Iran, North Korea and other nations who are seeking or have succeeded in obtaining or developing nuclear weapons. Neither Iran nor North Korea was invited to attend the session, which the Obama administration billed as the largest gathering of world leaders on U.S. soil since the U.N. founding conference in San Francisco in 1945.

The leaders agreed to hold a followup nuclear security summit in South Korea in 2012.

In a concluding news conference, Obama said he was confident China would join other nations in pressing for tough new sanctions on Iran for continuing to defy the international community in seeking such weapons.

"Words have to mean something. There have to be some consequences," Obama said.

Chinese President Hu Jintao met with Obama on Monday, then on Tuesday gave a speech to the group calling for "effective" measures to safeguard nuclear weapons and materials. But he stopped short of mentioning Iran's program.

Iran denies it intends to build an atomic bomb, and despite widespread concern about its intentions, Obama is having difficulty getting agreement on a new set of U.N. sanctions against the country. He said Tuesday that Hu had assured him that China would participate in drafting sessions at the United Nations on strong sanctions.

Throughout his news conference, Obama set a realist's tone about the role of the United States in world affairs. That is, it is necessary for the U.S. to lead on matters such as nuclear security. But on getting countries in the Mideast to agree to peace, on getting the Chinese to move to a market-based currency, on getting Iran and North Korea to play by international rules on nuclear compliance, on getting countries to live up to their fresh pledges on nuclear security, Obama repeatedly said the U.S. can't be the enforcer of world order. Countries must act in line with their own interests, he said.

As applied to Iran, this means U.N. and other sanctions are designed to change Iran's calculations about what it would gain from giving up its nuclear ambitions, he said.

Asked about steps that have been taken against North Korea, Obama conceded that "sanctions are not a magic wand." Still, he said he hoped the pressure could lead North Korea's leaders to return to nuclear disarmament talks that they abandoned before Obama took office.

In a joint work plan spelling out specific actions to be taken, the summit countries said they would "work together to achieve universality" of the International Convention for the Suppression of Acts of Nuclear Terrorism, but there was no mention of specific additional countries formally ratifying the convention. They also underscored the importance of a 2005 amendment to the Convention on the Physical Protection of Nuclear Material.

Daryl Kimball, head of the Arms Control Association, applauded the summit, which he said was an important step.

"This is the highest level of attention this issue has gotten ever," Kimball said.

But Sen. Jon Kyl, a Republican critic of Obama's nuclear policies, said the conference was a disappointment.

"The summit's purported accomplishment is a nonbinding communique that largely restates current policy and makes no meaningful progress in dealing with nuclear terrorism threats or the ticking clock represented by Iran's nuclear weapons program," Kyl said.

As an example of the collective action called for by 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. Each country 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 combined 68 tons of U.S. and Russian plutonium represents enough for about 17,000 nuclear weapons. The deal was signed Tuesday at the summit by Secretary of State Hillary Rodham Clinton and Russian Foreign Minister Sergey Lavrov.