WASHINGTON (Agencies) US President Barack Obama Sunday warned that nuclear-armed terrorists would pose the single biggest threat to the United States, warning that Al-Qaeda would not hesitate to use a nuclear weapon against the US. Obama conjured up the horrific possibility of a nuclear detonation in New York City, London or Johannesburg, and the serious global economic, political and security trauma that would result, to characterise the gravity of the threat. Obama, speaking on the eve of an unprecedented 47-nation summit in Washington aimed at thwarting nuclear terrorism, said he expected enormous progress at the conference towards the goal of locking down loose nuclear material worldwide. The central focus of this nuclear summit is the fact that the single biggest threat to US security - short-term, medium-term and long-term - would be the possibility of a terrorist organisation obtaining a nuclear weapon, Obama told reporters. This is something that could change the security landscape of this country and around the world for years to come, he said, warning of the potential consequences if a nuclear bomb were detonated. 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. He added, 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. Obama, who last week signed a landmark disarmament treaty with Russia and laid out a new US nuclear strategy limiting how Washington could use atomic weapons, said he was confident that the nuclear summit would garner important progress. I feel very good at this stage in the degree of commitment and sense of urgency that Ive seen from the world leaders so far on this issue, Obama said. We think we can make enormous progress on this. US officials hope nations at the summit will agree a series of their own security steps for their own nuclear material, and help pay to put the stocks of less well-off countries under lock and key. They also expect some leaders to unveil specific actions, similar to Chiles decision to ship a stock of highly enriched uranium to the United States.