VIENNA (Reuters) - Thieves have made off with a truck in Mexico carrying a dangerous radioactive material used in medical treatments, the United Nations nuclear watchdog said on Wednesday.

The flag of the International Atomic Energy Agency (IAEA) flies in front of its headquarters during a board of governors meeting in Vienna November 28, 2013. REUTERS/Heinz-Peter Bader

The International Atomic Energy Agency, the Vienna-based U.N. nuclear body, did not give details on how much of the radioactive source, cobalt-60, was in the truck at the time.

Cobalt-60 can potentially also be used to make a so-called “dirty bomb”, where conventional explosives are used to disperse radiation from a radioactive source. But the IAEA made no mention of this in its statement.

The IAEA 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 in Tepojaco near Mexico City on Monday.

“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.

More than 100 incidents of thefts and other unauthorized activities involving nuclear and radioactive material are reported to the IAEA annually, the U.N. agency said earlier this year.

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.

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.

But a “dirty bomb” is seen as a “high probability, low consequence act” with more potential to terrorize 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 bombs.

“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.