The International Atomic Energy Agency (IAEA) said Wednesday it had been informed by Mexican authorities that the truck was stolen from a service station in Tepojaco near Mexico City several days earlier.

The vehicle was transporting cobalt-60, used for radiotherapy treatment to combat cancer, from a hospital in the northern city of Tijuana to a radioactive waste storage centre when it was stolen on Monday.

"At the time the truck was stolen, the source was properly shielded. However, the source could be extremely dangerous to a person if removed from the shielding, or if it was damaged," an IAEA statement said.

It did not give details on how much radioactive material was in the vehicle when it was seized.

The UN's nuclear agency has offered to assist Mexican authorities who have launched a search in six Mexican states and in Mexico City for the white Volkswagen Worker truck.

'Dirty bomb' risk

Cobolt-60, the most common radioactive isotope of cobalt, is highly dangerous to humans if not handled properly.

It "has figured in several serious source accidents including fatalities because the material was obtained and handled by people who were not aware of its danger," said Mark Hibbs, analyst at the Carnegie Endowment for International Peace.

"There are many thousands of these sources worldwide. There is in most countries a regime to keep them safe and secure, but if they are stolen or lost, many people would be unaware that the radioactive contents locked up inside could threaten their lives," Hibbs added.

Apart from its many applications in industry and in radiotherapy in hospitals, experts warn it could also be used to create a so-called "dirty bomb" - an explosive device able to spread radioactive material over a wide area.

At a nuclear security summit in 2012, IAEA Director General Yukiya Amano singled out cobalt-60 among radioactive sources that could be used for such devices.

"These materials, such as cobalt-60, could be used along with conventional explosives to make so-called dirty bombs. A dirty bomb detonated in a major city could cause mass panic, as well as serious economic and environmental consequences," Amano said, according to a copy of his speech.

ccp/rc (AFP, Reuters, AP)