The United Nations' nuclear watchdog agency is urging countries to tighten controls on radioactive materials in the wake of the terrorist attacks in the United States. The International Atomic Energy Agency held an emergency meeting in Vienna Friday to warn nations that they must prevent terrorists from obtaining such materials, and beef up security around nuclear plants.

IAEA Director General Mohamed ElBaradei told delegates at the meeting that the willingness of terrorists to commit suicide and inflict massive damage makes the possibility of a nuclear-related attack much more likely now than before September 11.

IAEA spokeswoman Melissa Fleming has said the attacks in New York and Washington that day served as a wake-up call to the world.

"We need to look at everything we can do to step up preventive measures, in both the security of nuclear facilities against a potential aerial attack, as well as the possibility that nuclear materials could be diverted or stolen by a terrorist," she said.

The IAEA says the most devastating scenario, that terrorists will actually explode an atomic bomb, is the least likely possibility, but one that should not be ruled out. It is calling on countries that possess the bomb to ensure that security at their nuclear weapons facilities is at its tightest.

IAEA spokeswoman Fleming says her agency is much more concerned that terrorists could obtain radioactive material from such sources as hospitals or university laboratories to build what she describes as a "dirty bomb."

"These are the sources that, actually, are least secured, if secured at all, and this is why we are concerned. The way that would work is just shrouding them with conventional explosives. It wouldn't be such a difficult thing, although, certainly, physically dangerous for the person handling it. It would also not be nearly as devastating in terms of fatalities, but it would certainly cause the kind of panic that terrorists are out to achieve, as well as potential economic and environmental devastation," she said.

The IAEA also warns that nuclear plants and reprocessing facilities are vulnerable, especially if a fuel-laden jetliner slams into a nuclear reactor.

The United States has already acknowledged this threat by establishing no-fly zones around its nuclear facilities. France has stationed anti-aircraft missiles around its biggest nuclear facility.