ASTANA, Kazakhstan (AFP) - Authorities in Kazakhstan are on high alert after a container holding the radioactive substance caesium-137 disappeared in the west of the country, police said Tuesday.

A police spokesman for the Mangistau region said the material - commonly used for military and medical purposes - appeared to have fallen off a vehicle that was 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 learnt it had gone missing last Wednesday.

High 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 security services, emergency response workers and the military have been involved in efforts to find the container, which weighs some 50-60kg.

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.