Security should be tightened urgently to prevent radioactive material falling into the hands of terrorists, the UN's nuclear watchdog has said.
Many say that radioactive sources are not properly guarded
Additional measures were necessary "in view of recent reports about terrorist plans to build and deploy" devices capable of dispersing radioactivity, said the head of the International Atomic Energy Agency (IAEA).
Mohamed ElBaradei told a gathering of hundreds of nuclear experts in the Austrian capital Vienna that many countries lacked the resources to control radioactive sources.
United States Energy Secretary Spencer Abraham promised more aid to help countries track down lost or stolen radioactive material.
The US would give $3m next year to help poorer countries secure potentially dangerous radioactive sources, he announced.
"The threat requires a determined and comprehensive
international response... it is our responsibility to determine how to prevent
such an attack in the first place," he said.
It is the first global conference looking specifically at the threat posed by "dirty bombs" - devices which combine a simple explosive with any radioactive material.
There has been rising public concern since the 11 September attacks in the US about terrorists using radioactive material to create such a weapon .
Although a dirty bomb would not immediately kill as many people, it is feared that the threat of exposure to radiation and long-term illness might spark mass panic, and that urban areas could become uninhabitable for years.
Focus of concern
According to the IAEA, traditional security measures have often been limited to preventing accidental access or petty theft.
"Given the apparent readiness of terrorists to disregard their own safety, the personal danger from handling powerful radioactive sources can no longer be seen as an effective deterrent," Mr ElBaradei said.
" In view of recent reports about terrorist plans to build and deploy radiological dispersion devices and given the inadequacy of source control... it is clear that additional security measures are urgently needed.
"This concern has been the focus of the international community in the last 18 months," he said.
The IAEA also warned that the danger of radioactive material disappearing from official records was especially acute in the countries of the former Soviet Union.
Mr ElBaradei told the Vienna conference that there had been more than 280 confirmed cases of criminal trafficking of radioactive material, but "the actual number of cases may be significantly larger than the number reported to the agency".
Radioactive material is extensively used around the world in hospitals, research institutes and industrial sites such as oil drilling complexes.
Radiotherapy machines used to treat cancer are a common source, but radioactive material is also present in a wide variety of other applications.
The meeting of 600 scientists and government officials - hosted by the IAEA and co-sponsored by the US and Russian governments - aims to increase security and public awareness.