* Radioactive sources used in hospitals around world
* But could also be used in so-called dirty bomb
* No detail on how much cobalt-60 was in stolen truck
Thieves have made off with a truck in Mexico carrying a dangerous radioactive source used in medical treatments, a material that could also provide an ingredient for a so-called "dirty bomb".
The U.N. nuclear agency said it had been informed by Mexican authorities that the truck, which was taking cobalt-60 from a hospital in the northern city of Tijuana to a radioactive waste-storage centre, was stolen near Mexico City on Monday.
Apart from peaceful medical and industrial uses, experts say cobalt-60 can also be used in a dirty bomb in which conventional explosives disperse radiation from a radioactive source.
The International Atomic Energy Agency, which has stepped up calls on member states to tighten security to stop nuclear and radioactive materials falling into the wrong hands, made no mention of any such risk in its statement on Wednesday.
The IAEA also did not give details on how much radioactive material was in the vehicle when it was seized.
"At the time the truck was stolen, the (radioactive) source was properly shielded. However, the source could be extremely dangerous to a person if removed from the shielding, or if it was damaged," the IAEA statement said.
The IAEA has offered to assist Mexican authorities, who it said were searching for the material and had alerted the public.
Cobalt-60 - the most common radioactive isotope of cobalt, a metal - has many applications in industry and in radiotherapy in hospitals. It is also used for industrial radiography to detect structural flaws in metal parts, according to the U.S. Environmental Protection Agency.
The EPA said exposure to gamma radiation from cobalt-60 results in an increased risk of cancer.
"Cobalt-60 has figured in several serious accidents, some of them fatal," nuclear expert Mark Hibbs of the Carnegie Endowment think-tank said. "If dispersed, cobalt-60 or other radioactive source material could cause radiation poisoning locally."
More than 100 incidents of thefts and other unauthorised activities involving nuclear and radioactive material are reported to the IAEA annually, the U.N. agency said earlier this year.
It is rare, however, that it makes any such incident public.
Because radioactive material is regarded as less hard to find and the device easier to manufacture, experts say a so-called "dirty bomb" is a more likely threat than a nuclear bomb in any attack by militants.
DIRTY BOMB COULD CAUSE "PANIC"
Experts describe the threat of a crude fissile nuclear bomb, which is technically difficult to manufacture and requires hard-to-obtain bomb-grade uranium or plutonium, as a "low probability, high consequence act" - unlikely but with the potential to cause large-scale harm to life and property.
A "dirty bomb" is seen as a "high probability, low consequence act" with more potential to terrorise than cause large loss of life.
At a nuclear security summit in 2012, IAEA Director General Yukiya Amano specifically 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.
The veteran Japanese diplomat has repeatedly urged member states to ratify an amendment to the Convention on the Physical Protection of Nuclear Materials to expand its coverage to include domestic use, transport and storage. It now covers only physical protection of materials in international transport.