By Manish Chand

Washington, April 13 (IANS) The two-day Nuclear Security Summit began Monday as US President Barack Obama welcomed leaders of 46 countries who have gathered in the American capital to ensure bomb-making materials do not fall into the hands of terrorists.

Obama welcomed world leaders, including Indian Prime Minister Manmohan Singh and Chinese President Hu Jintao, shaking their hands and posing for photograph as they entered the Walter E. Washington Convention Centre that has become a virtual security fortress.

The summit is the largest gathering of world leaders called by an American president since the iconic Franklin D. Roosevelt organised the 1945 meeting in San Francisco that gave birth to the United Nations.

The summit will culminate with a joint communique Tuesday that will spell out a specific plan to secure all loose bomb-usable materials around the world within four years. According to an estimate, there is enough weapon-grade material floating around the globe that could help build 120,000 nuclear bombs.

On the eve of the summit, Obama Sunday described the possibility of a terrorist organisation obtaining a nuclear weapon, "the single biggest threat to US security, both short-term, medium-term and long-term".

The summit aims at developing a detailed plan to secure nuclear materials, prevent their smuggling and deter, detect and disrupt attempts at nuclear terrorism.

Manmohan Singh is among the world leaders invited for the summit that includes the major, minor and aspiring nuclear powers. Iran and North Korea have been excluded for their defiant violations of their Nuclear Non-Proliferation Treaty obligations.