Authorities in Kazakhstan are on high alert after a container holding the highly radioactive and dangerous substance caesium-137 disappeared in the west of the country, police said Tuesday.

A police spokesman for the Mangistau region said the material -- used for medical purposes and also a by-product of nuclear explosions and reactors -- appeared to have fallen off a vehicle transporting it.

"The container with the radioactive isotope caesium-137 has not been found so far," police spokesman Azamat Sarsenbayev told AFP, adding that authorities discovered it had gone missing last Wednesday.

Exposure to caesium-137, which has a half-life of 30 years, can result in severe burns or even death, and locals have been warned not to open the container if they find it.

The country's security services, emergency response workers and the military have been involved in efforts to find the container, which weighs some 50-60 kilogrammes (110-130 pounds).

The origin of the missing material was not revealed by authorities in Kazakhstan, which inherited nuclear warheads and a weapons test site when the Soviet Union collapsed.

The UN atomic watchdog, the International Atomic Energy Agency, to which such incidents are normally reported, had no immediate comment to make on Tuesday.

Caesium-137 is just one of many radioactive substances used in hospitals, universities and industry worldwide. Others include iridium-192, americium-241 -- used in smoke detectors -- and cobalt-60.

Every year dozens of cases of loss, theft or unauthorised activity are reported to the IAEA, and there have been numerous incidents of these substances causing serious illness and fatalities.

But the big worry is that extremists could get hold of the materials and use them in a "dirty bomb" -- a device whereby conventional explosives disperse radioactive materials.

Although the damage and loss of life caused by such a "dirty bomb" would be a fraction of that unleashed by a fission or fusion atom bomb, it could still cause mass panic in an urban area.