WASHINGTON — PresidentBarack Obama’snuclear summit of 47 world leaders met two goals as it ended Tuesday: reaching international consensus on the need to keep weapons-grade nuclear materials out of the hands of terrorists, and re-establishing U.S. leadership on nonproliferation.

Several nations agreed to dispose of weapons-grade uranium, end plutonium production, tighten port security and other voluntary steps. All participants endorsed Obama’s call to secure vulnerable nuclear materials in four years, and agreed to seek further cooperation even as they stopped short of any enforceable international agreement.

“”We have seized this opportunity,”” Obama said in a news conference closing the summit. As a result, he said, “”the American people will be safer, and the world will be more secure.””

Obama conceded that when it comes to enforcement, “”we’re relying on good will.””

The unanimous communique expressed support for security agreements “”that will not infringe upon the rights of States to develop and utilize nuclear energy for peaceful purposes and technology.”” Representatives of participating countries plan to reconvene in December for an update. Another leaders’ summit is planned for 2012 in South Korea.

Despite the administration’s effort to keep the summit focused on the general theme of nuclear security rather than individual countries, critics were vocal about Israel’s nuclear arsenal and Iran’s nuclear program.

The head of Saudi Arabia’s delegation told the gathering that “”Israel’s possession of nuclear weapons constitutes a fundamental obstacle to the achievement of security and stability in the Middle Eastern region.””

Obama dodged a question about whether he would call specifically on Israel to declare its nuclear program and sign the Nuclear Non-Proliferation Treaty.

“”As far as Israel goes, I’m not going to comment on their program,”” Obama said. “”Consistently, we have urged all countries to become members of the NPT.””

On another topic, when asked what international sanctions against Iran can achieve, Obama said that sanctions “”aren’t a magic wand”” but could “”change the calculus.””

The gathering provided Obama an opportunity to recast how other nations see the U.S. on nuclear issues, in contrast to the Bush administration, whose strategy had included developing new nuclear weapons and expanding circumstances under which they could be used.

It also was a chance for Obama to show U.S. voters another side of him. The public’s impression of him could improve if they view him as effectively exerting leadership on the world stage. The summit featured imagery of Obama surrounded by nearly four dozen other world leaders at the largest international conference since the World War II era, and produced results.

China said it will work with the U.S. and Europeans on new U.N. sanctions against Iran for its nuclear program. Last week Obama signed a new U.S.-Russia arms reduction treaty.

Gary Samore, the arms control and nonproliferation coordinator for the National Security Council, said of all the agreements: “”We used the summit shamelessly as a forcing event to ask countries to bring house gifts. Almost every country came with something new.””

“”This event has some political dimension to it above and beyond the actual content,”” saidLeonard Spector, the deputy director of the James Martin Center for Nonproliferation Studies. “”There’s a solidarity, a recognition this is a problem we all confront and we should do our best.””

More than 2,000 tons of plutonium and highly enriched uranium for civilian and military use are in dozens of countries, summit documents said, with 18 documented cases of theft or disappearance.

Obama said Tuesday that only an apple-sized amount of plutonium in the hands of terrorists could kill or injure hundreds of thousands of people. “”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 participants Tuesday. “”Were they to do so, it would be a catastrophe for the world.””

The president described “”a cruel irony of history”” two decades after the Cold War, in which “”the risk of a nuclear confrontation between nations has gone down, but the risk of nuclear attack has gone up.””

The nation of Georgia disclosed that Georgian authorities last month prevented an attempt by a criminal gang to smuggle highly enriched uranium.

“”The Georgia Ministry of Interior has foiled eight attempts of illicit trafficking of enriched uranium during the last 10 years, including several cases of weapons grade enrichment,”” its statement said. “”Criminals associated with these attempts have been detained. The most recent case of illicit trafficking was an attempt that failed of highly enriched uranium in March this year.””

The summit produced a number of concrete agreements, including a decision by Ukraine to dispose of all of its estimated 90 kilograms of highly enriched uranium by 2012, and an accord under which the U.S. and Canada will help Mexico convert a research reactor to lower enriched fuel. Chile also has said it is giving up highly enriched uranium.

Russia announced at the summit that it will shut down its last plutonium production reactor.

The U.S. and Russia signed on Tuesday a long-awaited agreement under which the former Cold War rivals each will dispose of 34 metric tons of weapons-grade plutonium and seek to get rid of additional material.

“”The initial combined amount, 68 metric tons, represents enough material for approximately 17,000 nuclear weapons,”” said a joint announcement of the agreement signed by Secretary of StateHillary Rodham Clintonand Russian Foreign MinisterSergei Lavrov. “”Both countries aim to begin actual disposition by 2018, after the necessary facilities are completed and operating.””

Russian PresidentDmitry Medvedevcalled the nuclear summit “”a complete success.””

Many nations don’t see the threat of nuclear terrorism with the same urgency as the U.S. does.

Yet, saidMatthew Bunnof Harvard University’s Belfer Center for Science and Technology: “”Pretty much every country where there are nuclear weapons, HEU (high-enriched uranium) or plutonium has more to do to protect them against the kinds of capabilities that thieves or terrorists have shown they can put together.””

Securing bomb-grade nuclear materials from theft involves storing it inside high-security compounds protected by armed guards, gates and fences, closed-circuit TV systems and penetration-detection alarms.

Individual countries are responsible for ensuring that their fissile stocks are protected. There are no binding international standards for the transportation and storage of bomb-making materials, in part because some nations see such requirements as infringements on their sovereignty.

Even the U.S., with what are considered the world’s best-guarded nuclear weapons facilities, has had problems with security and accounting systems.

(Warren P. Strobelcontributed to this article.)