The leaders or top officials from 53 countries, plus Interpol and three other international organisations, officially began the event with a working dinner hosted by South Korean President Lee Myung-Bak.

The leaders are assessing developments since Obama hosted the first such summit inWashingtonin 2010. In a statement, summit organisers hailed "substantial progress" since then, "based on a shared understanding of the danger of the threat of nuclear terrorism."

Obama in 2009 outlined his vision of a world without nuclear weapons.    Since theWashingtonsummit, he said in a speech inSeoulearlier Monday, "thousands of pounds of nuclear material have been removed from sites around the world -- deadly material that is now secure."

Copyright AFP (Agence France-Presse), 2012