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, local police have said.

A police spokesman for the Mangistau region said on Tuesday that 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," Azamat Sarsenbayev told the AFP news agency, 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.

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The country's security services, emergency response workers and the military have been involved in efforts to find the container, which weighs about 50kg to 60kg.

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 International Atomic Energy Agency (IAEA), the UN atomic watchdog, made no immediate comment 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.

Governments fear lost materials could be used to make a "dirty bomb" - a device whereby conventional explosives disperse radioactive materials.

Source: Agencies