Leaders of more than 50 nations have reaffirmed their commitment to reducing the world's stockpiles of nuclear weapons.

U.S. President Barack Obama told attendees at the start of Tuesday's final session of the two-day Nuclear Security Summit in Seoul that the international community must make “a serious and sustained effort” to reduce the world's stockpiles of nuclear weapons.

“There are still too many bad actors in search of these dangerous materials, and these dangerous materials are still vulnerable in too many places. It would not take much, just a handful or so of these materials to kill hundreds of thousands of innocent people, and that's not an exaggeration, that's the reality that we face.”

Mr. Obama acknowledged the progress that has been made since he hosted the 2010 summit in Washington, and noted that the number of nations participating this year has grown to more than 50.

He said the result will be a “larger global architecture” that will also allow the international community to “safely and effectively” pursue the peaceful use of nuclear energy.

But a statement issued Tuesday at the end of the summit offered no substantial plan for achieving that goal. Martine Letts, a high-ranking official at a Australian research center tells VOA the forum in Seoul is just one small part of what she calls a “a broad enterprise” in securing nuclear material.

“Summits are great in declaring lofty ideals…obviously the devil lies in the details. In particular, the capacity of nations to control such material within their borders. And there are many nations which have nuclear material, not just the known nuclear weapons states, where the security of that material is an issue of considerable concern. So declarations aside, what is important is to make sure those countries have the capacity and the political will and the governance arrangements to control those materials.”

The summit has been overshadowed by North Korea's recent announcement that it will launch a satellite next month. Many Western nations believe the launch is actually a test of a long-range missile.

Shortly after arriving in South Korea, Mr. Obama warned Pyongyang the launch would jeopardize the recent agreement in which the U.S. would provide the regime food in exchange for the North freezing its nuclear program. North Korea issued a statement Tuesday saying it would go ahead with the rocket launch, and called the U.S. president's remarks “confrontational.”

In his opening remarks at Tuesday's opening session, Japanese Prime Minister Yoshihiko Noda said North Korea would be violating United Nations Security Council resolutions if it went ahead with the launch.

Letts says the isolated regime used the satellite launch to hijack the nuclear summit and its stated agenda. But Ben Rhodes, Mr. Obama's deputy national security advisor, insisted during a press conference that North Korea has not been the summit's central focus.

“What this is about is preventing an act of nuclear terrorism, and in the aftermath of September 11th, you'll recall that the great concern of policymakers in Washington and around the world was the potential for terrorist groups like al Qaida to obtain a nuclear device and explode it in an American city. And they'd expressed their interest in doing that, and we also knew that there was significant amounts of nuclear material that was not adequately secured around the world, and that there were smuggling networks that could potentially be exploited as well, for terrorist groups to obtain this material.”

The United States and leading European suppliers of medical isotopes — Belgium, France, and the Netherlands — announced Monday they will move away from the use of weapons-grade highly enriched uranium (HEU) to a less potent form of the material.

U.S. Energy Secretary Steven Chu says the four-nation deal reduces the chance of the material falling into the wrong hands.

“Each reactor may look small but when you look at the plutonium being shipped and secured, the amount of highly enriched uranium being secured, now nearly 200 research reactors slowly being converted to this. This is something which I would not characterize this as small stuff.”

The Fissile Materials Working Group, an international coalition of nuclear experts, says the agreement marks an important step forward towards securing the global stockpile of nuclear materials, but insists bolder action is needed.