A two-day global summit on nuclear security, being attended by Presidents and Prime Ministers from 47 countries including leaders of India and Pakistan opens in Washington today. <br/><br/>Top topic of the meeting is how to safeguard nuclear materials, especially the fissile materials that might be used in making dirty bomb, such as plutonium and highly-enriched uranium as well as the more widespread sources of radioactive substances, from falling into the hands of terrorists. <br/><br/>President Barack Obama, who is hosting the summit, has set before him the goal of obtaining an agreement to secure all vulnerable nuclear materials within four years. <br/> <br/> Speaking ahead of the summit, he said, the biggest threat to world security is the possibility of a terrorist organisation obtaining a nuclear weapon. <br/><br/>The President, who held bilateral meetings with leaders of nuclear rivals, India and Pakistan yesterday, told reporters, nuclear weapons are not simply an issue for the US and Russia. <br/><br/>They threaten the common security of all nations. <br/><br/>Nuclear weapon in the hands of a terrorist is a danger to the people everywhere, he warned. <br/> <br/>Mr. Obama also told reporters that terrorist organisations like al-Queda are trying to secure nuclear weapons and if they succeed, it could change the global security landscape for years to come. <br/><br/>He said, terrorists would not hesitate to use those weapons. Incidentally, the Washington nuclear security summit would be third element of President Obama's nuclear initiatives this season that began with the unveiling of his Administration's nuclear strategy, the Nuclear Posture Review last week. <br/><br/>This identified nuclear proliferation – the spread of nuclear weapons and the danger that they might fall into the hands of terrorist groups – as now the key nuclear threat to America's security. <br/><br/>Step two was his meeting with Russian President Dmitry Medvedev in Prague that got the strategic arms reduction process back on track. <br/><br/>Step three is the nuclear security summit in Washington. <br/><br/>All three events are aimed at strengthening Mr. Obama's hands as he heads into step four, the review of the Nuclear Non-Proliferation Treaty scheduled for next month in New York. <br/>