"The central focus of this nuclear summit is the fact that the single biggest threat to US security, both short term, medium term and long term, would be the possibility of a terrorist organisation obtaining a nuclear weapon," Obama told reporters in between his series of bilateral meetings with world leaders including Prime Minister Manmohan Singh.
"So far today I've already met with Prime Minister Singh of India, as well as President of Kazakhstan, and now we are meeting with the President of South Africa," Obama said.
Later Obama met Pakistan Prime Minister Yousuf Raza Gilani at the Blair House across the White House, where all other meetings were held.
The US President said that nuclear terrorism could alter US security landscape including that of the entire world for years to come.
"If there was ever a detonation in New York City, or London, or Johannesburg, the ramifications economically, politically, and from a security perspective would be devastating," he asserted.
"We know that organisations like al Qaeda are in the process of trying to secure a nuclear weapon -- a weapon of mass destruction that they have no compunction at using," he said.
"Unfortunately, we have a situation in which there is a lot of loose nuclear material around the world. And so the central focus for this summit is getting the international community on the path in which we are locking down that nuclear material in a very specific time frame with a specific work plan," Obama said.
"One of the things that I'm very pleased about is that countries have embraced this goal and they're coming to this summit, not just talking about general statements of support but rather very specific approaches to how we can solve this profound international problem," he said.
Obama lauded South Africa's decision to abandon its nuclear weapons programme.
"I wanted to especially single out South Africa because South Africa is singular in having had a nuclear weapon programme, had moved forward on it, and then decided this was not the right path; dismantled it; and has been a strong, effective leader in the international community around non-proliferation issues".
"South Africa has special standing in being a moral leader on this issue. And I wanted to publicly compliment President Zuma and his administration for the leadership they've shown. And we are looking forward towards the possibility of them helping to guide other countries down a similar direction of non-proliferation," Obama said.