She recalled that terrorists had attacked the World Trade Center in New York in 1993 with a van loaded with conventional explosives.

Holgate told reporters at a meeting in Vienna of the UN nuclear watchdog International Atomic Energy Agency (IAEA) that it was "not widely shared and understood" how risky the current situation is, especially since terrorists would not necessarily need top-level scientists to build a bomb.

The nuclear threat remains the big one, and all too real, said Holgate, a senior member of the Washington-based Nuclear Threat Initiative (NTI) think tank and a former US Department of Energy official for disposal of plutonium.

She said the "raw material for nuclear terrorism is housed in hundreds of facilities in dozens of countries and inadequately secured."

"That's the central point of the Global Threat Reduction Initiative" which the United States and Russia have launched to repatriate highly enriched uranium (HEU) and to convert nuclear research reactors from HEU to low enriched uranium (LEU) use.

"We know nuclear theft is happening already," she said, saying that one institute in Russia has documented "23 attempts over eight years to steal nuclear bomb-making materials."

"We know these failed. We don't know how many succeeded and went undetected," Holgate said.

She also said she did not think terrorists had yet a nuclear weapon. "If terrorist organizations had been able to do this (obtain one), they would have used it by now," Holgate said.

The stakes are high.

"A nuclear device going off in any large city around the globe is going to kill millions of people," she said.

"The economic damage can be in the trillions (of dollars) and it can also be global," she said.

"This is in contrast to a dirty-bomb threat that tends to be hyped," she said about concern that terrorists could use conventional bombs with radioactive materials, contaminating areas with radiation rather than destroying them with the blast of an atomic bomb.

Holgate said a problem in making sure that nuclear materials are not lying where terrorists can get them is that there is "lack of acceptance" within the Russian government that "their material is not adequately secured and that there is a relationship between terrorism and these materials."

But she said the Russians seemed to be more aware of the threat since the Beslan school tragedy and a recognition of "weaknesses" in the Russian system, due to bribes and poor security.

The United States and Russia have produced most of the highly radioactive material now spread throughout the world.

Holgate said the United States and the then-Soviet Union gave out 20 tonnes of HEU in the 1950s and 1960s as part of the Atoms for Peace program for the peaceful uses of nuclear energy.

"Keeping track of where this HEU is now kilogram by kilogram is difficult." she said.

In addition, over 1,000 tonnes were created by the United States and the Soviet Union for their weapons programs, and there is no minute accounting for this.

William Potter, from the Monterey Institute of International Studies, a California-based think tank, said that in addition the Soviet Union and now Russia have some seven icebreaker ships which use nuclear fuel enriched to about 60 percent, Potter said.

HEU is uranium enriched to over 20 percent, but weapons grade uranium starts at 80 percent enrichment for the U-235 isotope.

Holgate said terrorists could do without the sophistication needed for small bombs. "A truck size is probably a more relevant size," since such a bomb could be made with lower levels of HEU.

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