U.S. President Barack Obama says 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 made the remarks Tuesday in Seoul at the opening session of the Nuclear Security Summit. The president acknowledged the progress that has been made since he hosted the 2010 summit in Washington, and noted that the number of nations participating in this year's summit 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.