A summit aimed at reducing the risk of nuclear terrorism around the world is to be held in the South Korean capital, Seoul.
The meeting involves representatives from 50 countries.
Ever since the collapse of the Soviet Union in 1991, there have been fears about nuclear materials falling into the hands of terrorists and extremists.
In April 2010, President Barack Obama convened a summit in Washington that set the ambitious goal of securing all vulnerable nuclear materials worldwide within four years.
Some progress has been made but progress has been patchy, in part because the initial goal set in 2010 was vague without a detailed timeline or work plan.
Now two years on, this Seoul summit will try to create new momentum.
The summit agenda is to be expanded to include a wide variety of radiological materials which terrorists could use to make a bomb that spreads radiological contamination rather than initiating a nuclear explosion.