UNITED NATIONS  Mohamed ElBaradei, the chief of the International Atomic Energy Agency, said in a speech on Monday that the number of reports of nuclear or radioactive material stolen around the world last year was “disturbingly high.”

Dr. ElBaradei, in his annual report to the General Assembly, said nearly 250 such thefts were reported in the year ending in June.

“The possibility of terrorists obtaining nuclear or other radioactive material remains a grave threat,” he said. “Equally troubling is the fact that much of this material is not subsequently recovered.”

Members of Dr. ElBaradei’s staff and outside experts cautioned that the amount of missing material remained relatively small. If all the stolen material were lumped together, it would not be enough to build even one nuclear device, they said.