Nations must keep better inventories of radioactive material being used for industrial and academic purposes to prevent it from falling into the hands of terrorists, a US official said Monday. The improved scrutiny was needed because of the possibility of a so-called "dirty bomb," said Steven Aoki, a counterterrorism official at the US Energy Department. "This is a major problem with a potentially large amount of radioactive material out there and unaccounted for," Aoki told an international conference on illicit nuclear trafficking. "We need to get hold of it and store it securely." About 300 officials from more than 70 countries are meeting at the conference this week to assess trafficking in nuclear and other radioactive material and to propose a new policy regarding containment.