Officials from 47 countries are meeting in Washington to discuss how to keep nuclear weapons out of the hands of terrorists.
On the eve of the two-day summit, U.S. President Barack Obama said that efforts by Al-Qaeda to acquire nuclear weapons posed the biggest security threat.
Obama's goal is to get nations to agree to secure all nuclear material within four years and to take specific steps to crack down on nuclear smuggling.
Secretary of State Hillary Clinton said the gathering would be the largest assembly of world leaders hosted by an American president since the 1945 San Francisco conference that founded the United Nations.
Also on the summit's agenda will be Iran's nuclear program which the West fears could hide a nuclear weapons program, and North Korea's nuclear weapons stockpile and exports of nuclear materials and technology.
On April 11, Obama held one-on-one talks with many leaders set to attend the conference including Nursultan Nazarbaev, the president of Kazakhstan, which Obama noted had benefited greatly by giving up its nuclear weapons.
compiled from agency reports