Thousands of devices containing radioactive material have been lost or stolen worldwide, the auditing arm of the United States Congress has warned.
Experts fear radioactive material may fall into the wrong hands
A report by the General Accounting Office (GAO) says missing materials originally used in medicine, industry and research could be used to make crude but deadly "dirty bombs".
Most of the devices reported missing were in Russia, the GAO said.
Its report accuses the US administration of not doing enough to help countries that pose a security risk.
The auditors expressed particular concern over hundreds of electricity generators spread across the former Soviet Union, which contain deadly radioactive material.
There are about 10 million devices containing radioactive substances in the United States and 49 the other countries that responded to a GAO survey.
Although there is limited information about the number of devices that have been lost or stolen, "it is estimated to be in the thousands worldwide", according to the report.
There are hundreds of generators containing radioactive strontium 90 in the former Soviet Union.
The generators were designed to power communications stations, often in remote areas - but many are no longer used and some are unaccounted for.
While each generator contains only small amounts of strontium 90, nuclear experts say there could be enough for a dirty bomb if several devices were stripped of their material.
The GAO criticises the US Energy Department - which is charged with securing radioactive containers - for spending most of the programme's money in the US, rather than in high-risk countries.
The International Atomic Energy Agency estimates that as many as 110 nations do not have adequate controls over radioactive devices.