A "dirty" bomb, or a conventional explosive combined with radioactive material, is "not very effective as a means of causing fatalities," said Richard Meserve, chairman of the US Nuclear Regulatory Commission.

"But it could have a psycho-social effect, and terrorists' greatest weapon is fear."

Top nuclear officials accompanying Meserve underlined that in the case of such an explosion entire city sections would have to be razed to the ground because of radiation contamination.

Concerns about terrorist attacks on the United States have soared since the devastating September 11 suicide strikes that left some 3,000 dead, particularly amid reports that the al-Qaeda network believed responsible for those attacks have been searching for nuclear materials on the black market.

"Today we know that radiological and nuclear attacks on the United States are not only possible, but there are enough screwballs out there willing to risk or even give their lives to use them," Senator Joe Biden said.

"Radiological attacks constitute a credible threat," confirmed Henry Kelly, president of the Federation of American Scientists, speaking to the Senate Foreign Relations committee that Biden chairs.

And "radioactive materials that could be used for such attacks are stored in thousands of facilities around the US," he said in prepared remarks.

Materials that could easily be stolen or lost from US research institutions "could contaminate tens of city blocks at a level that would require prompt evacuation and create terror in large communities" even if radiation casualties were low.

On the other hand, nuclear material that could be used to build nuclear weapons -- highly enriched uranium and plutonium -- is "very highly guarded," Meserve said.

The threat of a nuclear device was more likely to come from poorly protected fissile stockpiles abroad such as in Russia, he said. "This is an important challenge (and) the US should be engaged to ensure protection at the source."

US intelligence has reported that despite foreign assistance and its own efforts to heighten security, weapons grade nuclear material has gone missing from Russia, which Biden described as a "candy store for terrorists."

The single, most effective means of protecting the United States from these threats, Meserve said, is "to control materials at the source."

Los Alamos National Laboratory director Donald Cobb told the panel the United States was working with Russia to secure nuclear weapons and material, but acknowledged that the materials were the greatest danger.

"Nuclear weapons and weapon-usable materials tend to be focused in military applications under tight government oversight ... Radiological sources are more wide spread and have fewer controls."

But Kelly noted that the amount of radioactive sources discovered recently abandoned in North Carolina, or typically found in US food irradiation facilities or oil well logging systems would be enough to contaminate large swathes of a city, slightly increase cancer rates and force buildings to be demolished.

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