More than 100 countries around the world need to upgrade their security to prevent the theft of radioactive material that could be used to build a "dirty bomb", says a United Nations report.
Some of these countries, including the United States and many European Union nations,must also do more to recover supplies that are missing.
"What is needed is cradle-to-grave control of powerful radioactive sources to protect them against terrorism or theft," said Mohamed ElBaradei, head of the International Atomic Energy Agency (IAEA), the UN body which produced the report.
It is part of the group's expanded efforts since the attacks of September 11 to tighten security around radioactive material that could be used to make a crude but potentially devastating dirty bomb.
Such a device would use conventional explosives to spread radioactive material over a limited area.
While experts believe a dirty bomb would initially kill no more people than a conventional bomb, the dispersal of radioactive materials such as cobalt-60, strontium-90, caesium-137 or iridium-192 would lead to many others being exposed.
It would also create huge panic.
Because of the relative ease with which they can be made, dirty bombs have become something of an obsession, particularly in the US.
Fears were fuelled this month when officials said they had arrested an alleged American al Qaeda operative, Abdullah al Muhajir, who was said to be plotting to build and detonate a dirty bomb in Washington.
The IAEA report says priority must be given to helping countries to create and strengthen regulatory infrastructures to ensure that radioactive sources are properly registered and secured.
A total of 82 countries have asked the agency for help in this area.
