VIENNA/MEXICO CITY (Reuters) - Mexican police have found a truck they suspect was stolen by common thieves and carried a dangerous radioactive medical material the United Nations said could provide an ingredient for a "dirty bomb," a government official said.

The truck was found on Wednesday close to where it was stolen outside Mexico City, said the official, who asked not to be identified in line with policy. He could not confirm whether the cargo was aboard.

The truck was stolen on Monday while it was taking cobalt-60 from a hospital in the northern city of Tijuana to a radioactive waste-storage center, Mexican officials and the UN agency said earlier.

The vehicle was seized when the driver stopped at a gas station in the town of Temascalapa, 35 km (22 miles) northeast of Mexico City.

Mexican officials say the thieves were likely common criminals unaware of the truck's radioactive cargo. Truck hijacking is common in Mexico and the theft occurred in the State of Mexico, which is not a drug cartel stronghold.

"Our suspicion is that they had no idea what they had stolen. This is a area where robberies are common," Fernando Hidalgo, spokesman for the Hidalgo state prosecutor, said earlier.

Mexico's national nuclear safety commission CNSNS published photographs of the cargo as it was being prepared for shipment, showing a reinforced case containing the medical device, which holds the radioactive material and which looks like part of a car axle. The box is marked with the hospital's name and "radioactive materials."

Apart from peaceful medical and industrial applications, 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 prevent nuclear and radioactive materials from 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. Inside a teletherapy device, cobalt-60 is used to treat cancer.

"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 said in a statement.

Cobalt-60, the most common radioactive isotope of the 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.

DIRTY BOMB COULD CAUSE 'MASS PANIC'

In 2000, three people died in Thailand after a cobalt-60 teletherapy unit was sold as scrap metal and ended up on a junkyard. About 1,870 people living nearby were exposed to "some elevated level of radiation," according to an IAEA publication.

About the same time in Mexico, homes built with metal rods that had been contaminated by stolen cobalt were destroyed, a spokesman for the attorney general's office said.

"Cobalt-60 has figured in several serious accidents, some of them fatal," said nuclear expert Mark Hibbs of the Carnegie Endowment for International Peace think-tank. "If dispersed, cobalt-60 or other radioactive source material could cause radiation poisoning locally."

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 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 make, experts say a dirty bomb is a more likely threat than a nuclear bomb in a terrorist attack.

Experts say a dirty bomb carries more potential to terrorize than cause a 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.

"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.

(Additional reporting by Michael Shields in Vienna and David Alire Garcia and Alexandra Alper in Mexico City; Editing by Mark Heinrich, Simon Gardner, Doina Chiacu, Mohammad Zargham and Jackie Frank)