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President Barack Obama outlined the grave threat posed by vulnerable nuclear materials Tuesday during the opening of the Nuclear Security Summit's second day.

"Nuclear materials that could be sold or stolen and fashioned into a nuclear weapon exist in dozens of nations," Obama said to the delegations of the 47 nations attending the D.C. based summit. "Just the smallest amount of plutonium, about the size of an apple, could kill and injure hundreds of thousands."

The president urged international cooperation in confronting the danger.

"We have the opportunity as individual nations to take specific and concrete actions to secure the nuclear materials in our countries and to prevent illicit trafficking and smuggling," Obama said.

Obama stressed that international efforts should not be "a fleeting moment" but rather "part of a serious and sustained" mission.

Tuesday's summit marks the largest gathering of foreign diplomats on U.S. soil since a meeting on the establishment of the United Nations in 1945. In 2012, South Korea will host a summit on nuclear security as well.