At a news conference wrapping up the two-day summit, Mr. Obama cited steps taken by countries including Russia and other former Soviet states to eliminate some of the vulnerable vestiges of nuclear stockpiles from the Cold War era.

During the summit, the United States and Russia signed an update to a 2000 agreement intended to eliminate weapons-grade plutonium from their military programs.

Earlier, the United States, Canada and Mexico agreed to work to convert the fuel at Mexico's nuclear research reactor to a lower grade of uranium unsuitable for nuclear weapons, the White House announced.

The US President said the summit's final statement acknowledges the urgency and seriousness of the threat of nuclear terrorism and sets a goal for securing all the world's vulnerable nuclear materials in the next four years.

The communique calls for strengthening international institutions such as the United Nations and the IAEA, and recognizes the "fundamental responsibility" of nations to meet their international obligations regarding the Nuclear Non-Proliferation Treaty and other agreements, Mr. Obama said. He also noted that the communique stresses that progress on such issues requires international cooperation.

The US President convened the summit as part of a broader strategy to decrease the threat of nuclear weapons and technology from reaching terrorists. It was considered an unprecedented effort to rally global action on securing vulnerable nuclear materials.