The United Nations Agency responsible for atomic safety says it is likely that terrorist organizations will attempt to obtain nuclear materials for future attacks.

The issue of nuclear terrorism is the topic of a special session Friday, of the International Atomic Energy Agency, based in Vienna. In New York, Agency spokesman Gustavo Zlauvinen told reporters there are three scenarios in which terrorists could employ nuclear assets.

The first, and most dangerous, he said, would be the use of an actual nuclear weapon. Fortunately, he also believes that is the least likely because of the specialized equipment and highly technical expertise that would be necessary to assemble such a weapon.

Second, according to Mr. Zlauvinen, would be an attack on a nuclear power plant. He observed that, while most plants were designed to withstand the impact of a small airplane, it is not known if they would survive attacks from large jetliners such as those used in the September 11 attacks on the United States. The IAEA is now researching that question.

Mr. Zlauvinen said the third, and most possible scenario, is the mixture of radioactive materials into a conventional bomb producing a so-called "dirty bomb." "That scenario, as awful as it sounds, probably would not produce many deaths," he explained. "However, the psychological impact would be tremendous as well as the radiological contamination of the area involved. That would require millions of dollars and several years to finish the clean up."

Mr. Zlauvinen said the International Atomic Energy Agency will put a new emphasis on helping governments improve their security standards. Of particular importance, he said, is the provision of proper training and equipment to border guards to prevent the illegal international transport of nuclear materials.