Extremist groups that possess nuclear weapons or materials will be the "No. 1 security threat" facing the world in the coming years, the head of the International Atomic Energy Agency said Monday. Mohamed ElBaradei said the Vienna-based UN agency needs more money to properly protect nuclear sources and to keep potentially lethal substances out of the hands of such groups. "Deterrence does not work in the case of extremist groups... because if they were to acquire a nuclear weapon or powerful radioactive source, they would simply use it," ElBaradei said in a speech to diplomats and politicians in Austria's parliament. ElBaradei, in other comments, appeared hopeful the new US administration would make progress in resolving the international standoff surrounding Iran's nuclear program.