Discussions are taking place behind closed doors. The results of the talks will be announced during a news conference afterwards.

In his opening speech, U.S. President Barack Obama said the next nuclear security summit will take place in South Korea in 2012.

During the summit, world leaders are expected to make concrete commitments to ensure that their national stocks of separated plutonium and enriched uranium are not stolen, smuggled or sold to terrorists.

The summit comes after the signing of a new strategic arms reduction treaty by U.S. President Barack Obama and Russian President Medvedev in Prague last Thursday.

The two countries, which possess about 90% of global arsenals of nuclear weapons, have agreed to reduce the number of nuclear warheads to 1,550 on each side and the number of deployed and non-deployed delivery vehicles to 800 on each side.

President Barack Obama, center, poses for a group photo with the delegation heads attending the Nuclear Security Summit in Washington, Tuesday, April 13, 2010. There are left to right from the bottom row to the top, South Korean President Lee Myung-bak, French President Nicolas Sarkozy, Georgian President Mikhail Saakashvili, Brazilian President Luiz Inacio Lula da Silva, Finnish President Tarja Halonen, Kazakh President Nursultan Nazarbayev, President Barack Obama, Jordanian King Abdullah II, Philippines President Gloria Macapagal Arroyo, Chinese President Hu Jintao, Mexican President Felipe Calderon, Argentine President Cristina Fernandez, Armenian President Serge Sarkisian, Norwegian Prime Minister Jens Stoltenberg, Indian Prime Minister Manmohan Singh, Turkish Prime Minister Recep Tayyip Erdogan, Nigeria acting President Goodluck Jonathan, Ukraine President Viktor Yanukovych, South African President Jacob Zuma, Russian President Dmitry Medvedev, Swiss President Doris Leuthard, Chilean President Sebastian Pinera, Dutch Prime Minister Jan Peter Balkenende, Spanish Prime Minister Jose Luis Rodriguez Zapatero, Singapore Prime Minister Lee Hsien Loong, German Chancellor Angela Merkel, European Union President Herman Van Rompuy, Belgian Prime Minister Yves Leterme, Czech Prime Minister Jan Fischer, New Zealand Prime Minister John Key, Pakistani Prime Minister Syed Yusuf Raza Gilani, Moroccan Prime Minister Abbas El Fassi, Vietnamese Prime Minister Nguyen Tan Dung, Canadian Prime Minister Stephen Harper, Swedish Prime Minister Fredrik Reinfeldt, Poland's Ambassador to the U.S. Robert Kupiecki, Italian Premier Silvio Berlusconi, Malaysian Prime Minister Najib Razak, Japanese Prime Minister Yukio Hatoyama, Saudi Intelligence Chief Prince Muqrin bin Abdulaziz Al Saud, Algerian Foreign Affairs Minister Mourad Medelci, Egyptian Foreign Minister Ahmed Aboul Gheit, Sheik Mohamed bin Zayed Al Nahyan, crown prince of Abu Dhabi and deputy supreme commander of the UAE armed forces, Indonesian Vice President Boediono, Deputy Th.