“The possibility of terrorists obtaining nuclear or other radioactive material remains a grave threat,” ElBaradei said in an address to the UN General Assembly.

“Equally troubling is the fact that much of this material is not subsequently recovered,” he said.

However, there is not enough missing nuclear material to produce one actual atomic bomb, according to IAEA personnel and independent analysts.

The heightened figures might also result from better reporting of missing material by nations, according to the Times.

The potential for unrecovered material to be used in a radiological weapon, though, makes the potential existence of a market for such substances a cause for concern, experts said. “Radiation released by a ‘dirty bomb’ is more likely to cause panic than extended health dangers, according to Cristina Hansell of the James L. Martin Centre for Nonproliferation Studies in California, the Global Security Network quoted her as saying.

“What will kill you from a dirty bomb is the immediate explosion, not the radioactivity,” she said. “The increasing amount of sensitive material that appears to go missing seems to be quite a big problem,” Hansell added.

Sensitive material is stolen from locations around the world, though the former Soviet Union has been an area of particular concern due to the significant number of nuclear programmes once found in the communist superpower, GSN said.