Jewel Samad/AFP/Getty Images

President Barack Obama shakes hands with China's President Hu Jintao (third left) as Kazakhstan's President Nursultan Nazarbayev (far right) and Georgia's President Mikheil Saakashvili (far left) look on at the start of the first plenary session of the 2012 Nuclear Security Summit at the COEX Center in Seoul on March 27.

World leaders including President Obama have begun the two-day summit in South Korea, aimed at curbing the threat of nuclear terrorism.