WASHINGTON (AP) — World leaders have 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. But they offered few specifics for achieving that goal.

The threat of nuclear terrorism was the focus of a 47-nation, two-day summit that ended Tuesday — billed as the largest gathering of world leaders on U.S. soil since the U.N. founding conference 65 years ago.

But it was two nations that were not invited — Iran and North Korea — that attracted much of the attention.

Obama had called the conference to focus world attention on the threat of nuclear terrorism, a peril he termed the greatest threat facing all countries 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. The leaders agreed to hold a follow-up nuclear security summit in South Korea in 2012.

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,” Obama told the opening session, which convened under tight security at the Washington Convention Center.

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.

Russian President Dmitry Medvedev called the summit hosted by Obama a “complete success.”

Just days after Medvedev and Obama signed a major nuclear weapons treaty, the Russian leader praised the new atmosphere of cooperation and said it was starting to yield results.

“The environment has been changed and there have been direct results,” he said. “And I must say that I am glad that I was part of that.”

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.

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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.

In Tehran on Tuesday, Iran expressed doubts that China would back the U.S. push for new sanctions. Foreign Ministry spokesman Ramin Mehmanparast said he did not think Hu was signaling that with his comments.

Chinese Foreign Ministry spokeswoman Jiang Yu said in Beijing on Tuesday that China supports a “dual-track strategy,” combining diplomacy with the possibility of international sanctions against Iran, but the country believes “pressure and sanctions cannot fundamentally solve it.”

Medvedev, at a speaking engagement later in Washington, sought to temper U.S. expectations for biting sanctions against Iran.

“If nothing happens, we will have to deal with sanctions,” Medvedev said. “I do not favor paralyzing, crippling sanctions that make people suffer.”

Obama, throughout his news conference at the conference Tuesday, 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 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.

Syria, which is suspected by the U.S. and others of harboring nuclear weapons ambitions, also was not invited to the conference.

French President Nicolas Sarkozy, meanwhile, is suggesting the idea of an international court to try states that provide nuclear technology or materials to terrorist organizations. Sarkozy said Obama found the idea “useful” and suggested that aides work with the U.N. on the premise.

“Today we have no way of punishing a country that provides nuclear materials to a terrorist organization,” Sarkozy said.

As the summit drew to a close Tuesday, the participating countries said in a joint work plan spelling out specific actions to be taken that 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.

U.S. officials, meanwhile, 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.

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Associated Press writers Julie Pace, Mark Smith, Desmond Butler, Maria Sanminiatelli and Ben Feller contributed to this report.

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