Fifty-three world leaders will meet in the Netherlands on Monday for the third Nuclear Security Summit to discuss how to reduce nuclear risks to global security.
The US president, Barack Obama, proposed the summit to deal with nuclear materials, which he said posed "the most immediate and extreme threat to global security".
Today there are 25 countries that hold stocks of weapons-grade material.
It is not clear if new tensions between Russia and the international community over Ukraine will damage what has been, until now, good co-operation between former Cold War enemies.
Al Jazeera's Simon McGregor-Wood reports from London.