The UN 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 (IAEA), 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 yesterday.
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.