President Barack Obama says the 47-nation Nuclear Security Summit that concluded in Washington Tuesday is an important step toward shared effective leadership in securing vulnerable nuclear materials over the next four years.  A final summit communique lays out steps individual countries say they will take, although on a voluntary basis, aimed at reducing the chances that terrorist organizations will obtain nuclear materials.

On the final day of the summit, the president again sounded the main theme of the summit, that the world faces a grave threat 20 years after the Cold War ended, from terrorist organizations attempting to acquire nuclear materials or weapons.

Saying terrorist groups such as al-Qaida would surely use a nuclear weapon if they obtained one, President Obama said acting to eliminate loose nuclear materials will require a new way of thinking.

"All this, in turn, requires something else, which is something more fundamental," said President Obama. "It will require a new mindset - that we summon the will, as nations and as partners, to do what this moment in history demands."

President Obama presided over two sessions, both closed to the media, and a working lunch.

A final three-page communique commits participants to secure all nuclear materials by 2014 and lists 12 specific obligations, including a promise to maintain effective security of nuclear material they possess.  

Nations also pledge to:  prevent non-state actors from obtaining information or technology required to use nuclear material for malicious purposes, adhere to a list of best practices, and recognize a need for cooperation to effectively prevent and respond to incidents of illicit nuclear trafficking.

In a concluding news conference, President Obama was asked about the specific stance China's President Hu Jintao took in a bilateral meeting regarding efforts at the United Nations to achieve agreement on a new sanctions resolution against Iran in the U.N. Security Council.

Saying China was part of a serious discussion in the P5+1 group on a new sanctions regime, the president acknowledged that negotiations will not be easy.

"I think these negotiations can be difficult and I am going to push as hard as I can to make sure that we get strong sanctions that have consequences for Iran as it is making calculations about its nuclear program and that those are done on a timely basis," said Mr. Obama.

Earlier, Deputy National Security Adviser Ben Rhodes underscored steps to secure nuclear materials and reinvigorate the Nuclear Nonproliferation Treaty (NPT) and send a message to non-complying countries:

"A nation like Iran has a right under the NPT to nuclear energy," said Ben Rhodes. "And if they can demonstrate that their nuclear program is for peaceful purposes it can be brought into the framework of the NPT, and Iranians can benefit from peaceful nuclear energy.  If however Iran continues to fail to live  up to its obligations under the NPT they need to be held accountable."

Daryl Kimball,  Executive Director of the Arms Control Association says by the time of the next nuclear summit in 2012 (in South Korea), participants will be able to assess progress toward the goal of securing nuclear materials, but more is riding on a work plan nations have committed to.

"It really represents a patchwork of ideas and initiatives, and this work plan stitches those together  and establishes a common plan of action for all these countries that hopefully other countries will support in the years ahead," said Daryl Kimball.

Along with measures the U.S. will take, such as strengthening port security, specific announcements during the summit included one in which Ukraine formally committed to eliminate its stockpile of highly enriched uranium by 2012, along with similar steps by Canada and Chile.

Others include an agreement in which the U.S. and Russia pledge to eliminate a total of 68 tons of weapons-grade plutonium, and Mexico's decision to convert fuel in a research reactor to lower enriched uranium.  

The final day of the summit, President Obama held additional bilateral meetings with German Chancellor Angela Merkel, Turkish Prime Minister Recep Tayyip Erdogan, Argentina's President Cristina Fernandez de Kirchner.

President Obama announced that the next Nuclear Security Summit will be held in South Korea two years from now.

After starting the day leading the gathered world leaders in a moment of silence for the the president of Poland, his wife and 93 others killed in a plane crash in Russia, the White House announced that the president will travel to Poland on Sunday to attend the funeral of President Lech Kaczynski and Maria Kaczynska taking place in the city of Krakow.

Related report by Carolyn Presutti