President Obama says terrorists getting their hands on a nuke is one of the greatest threats the world faces.

Addressing world leaders at the nuclear security summit in Washington, DC today, Obama said we have "measurably reduced the risk" of terrorists obtaining nuclear material, but we cannot be complacent.

Obama announced that 102 nations have ratified a treaty on protection of nuclear material.

The President also announced that for the first time the U.S. will provide a public inventory of its nuclear stockpile. (Metro News Networks)