Presidents, prime minsters, and top officials from nearly 50 countries mingled Monday on the threshold of President Barack Obama's nuclear proliferation summit, the largest assembly a U.S. leader has hosted since the founding conference of the United Nations in 1945.

Obama wants world leaders to confront the threat of nuclear arms falling into the hands of terrorists — a specter he labels "the single biggest threat to U.S. security." And he's looking at the high-profile security forum here to help him reach his goal of ensuring that all nuclear materials worldwide are secured from theft or diversion within four years.

A few hours ahead of Obama's official Monday afternoon welcome for the guests at a convention center here, he sat down for a series of meetings with international visitors, including Jordan's King Abdullah II.

On the eve of the meeting, Obama said that nuclear materials in the hands of al-Qaida or another terrorist group "could change the security landscape in this country and around the world for years to come."

At an unofficial parallel conference of more than 200 international nuclear experts, participants said too many around the world don't share the concern that nuclear terrorism is an urgent threat.

"There is a great complacency among policy makers around the world that terrorist groups couldn't make a nuclear bomb," said Matthew Bunn of Harvard.

Pakistani physicist Pervez Hoodbhoy focused on his home region, where both Pakistan and India are building up their nuclear arsenals.

"Unfortunately, I do not see this concern either in Pakistan or India about nuclear terrorism," he said. "Both countries do not see the seriousness of this situation."

American nonproliferation expert Robert Gallucci told the conference he believes it's "probable" over time that terrorists will detonate an atomic weapon in a city somewhere, not necessarily in the United States or Europe.

Clearly with Pakistan and India in mind, the former U.S. diplomat said, "Any country that has suffered serious terrorist attacks, foreign or domestic, needs to take this threat seriously."

Although sweeping or even bold new strategies were unlikely to emerge from the two-day gathering, the president declared himself pleased with what he heard in warm-up meetings Sunday with the leaders of Kazakhstan, South Africa, India, and Pakistan.

"I feel very good at this stage in the degree of commitment and a sense of urgency that I have seen from the world leaders so far on this issue," Obama said. "We think we can make enormous progress on this, and this then becomes part and parcel of the broader focus that we've had over the last several weeks."

He was referring to what had gone before this, the fourth leg of his campaign to rid the world of nuclear weapons. The United States is the only country to use the weapons — two bombs dropped on Japan to force its surrender in World War II.

The high-flying ambition, which the president admits probably will not be reality in his lifetime, began a year ago in Prague when he laid out plans for significant nuclear reductions and a nuclear-weapons-free world.

In the meantime, he has approved a new nuclear policy for the United States, promising last week to reduce America's nuclear arsenal, refrain from nuclear tests, and not use nuclear weapons against countries that do not have them. North Korea and Iran were not included in that pledge because they do not cooperate with other countries on nonproliferation standards.

That was Tuesday, and two days later, on the anniversary of the Prague speech, Obama flew back to the Czech Republic capital where he and Russian President Dmitri Medvedev signed a new treaty that reduces each side's deployed nuclear arsenal to 1,550 weapons. Medvedev also arrives Monday to sign a long-delayed agreement to dispose of tons of weapons-grade plutonium from Cold War-era nuclear weapons — the type of preventive action Obama wants the summit to inspire.

Throughout the two-day gathering, Iran will be a subtext as Obama works to gain support for a fourth round of U.N. sanctions against Tehran for its refusal to shut down what the United States and many key allies assert is a nuclear weapons program. Iran says it only wants to build reactors to generate electricity.

In an interview on ABC's "Good Morning America," Medvedev agreed that Iran's nuclear program must be watched closely, but he said sanctions on the regime would have to be smart and effective because they often don't work.

"They should not lead to humanitarian catastrophe, where the whole Iranian community would start to hate the whole world," the Russian president said.

He rejected the idea of imposing sanctions on Iran's petroleum industry.

"I don't think on that topic we have a chance to achieve a consolidated opinion of the global community," Medvedev said.

Support from Medvedev and Chinese President Hu Jintao, who sees Obama privately Monday, is critical, but neither is firmly committed to a new sanctions regime.