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.<br /><br />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".<br /><br />Today there are 25 countries that hold stocks of weapons-grade material.<br /><br />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.<br /><br />Al Jazeera's Simon McGregor-Wood reports from London.
