Very low levels of radioactive iodine-131, which are not believed to pose a health risk, have been detected in the Czech Republic and elsewhere in Europe.
The International Atomic Energy Agency, the Vienna-based UN watchdog, said it was seeking to determine the source but that it they are not believed to have come from Japan's stricken Fukushima plant.
The Czech Republic nuclear security authority said it did not think it was the source, saying it could possibly be from the production of radiopharamceuticals.
Iodine-131, linked to cancer if found in high doses, can contaminate products such as milk and vegetables.
The IAEA said the Czech Republic's nuclear safety body had informed it that "very low levels" of iodine-131 had been measured in the atmosphere over the central European country in recent days.
"The IAEA has learned about similar measurements in other locations across Europe," the brief statement said.
"The IAEA believes the current trace levels of iodine-131 that have been measured do not pose a public health risk and are not caused by the Fukushima Daiichi nuclear accident in Japan," it added.
"The IAEA is working with its counterparts to determine the cause and origin of the iodine-131."
Iodine-131 is a short-lived radioisotope that has a radioactive decay half-life of about eight days, the IAEA said.
The Czech watchdog said it had detected iodine-131 at a number of monitoring stations since late October. It said there was no health risk from the iodine.
The IAEA said it did not know the source of the iodine-131 and would give more details when available. It did not give details of the other locations in Europe where levels had been detected.
Officials in Spain and Ukraine said they had not detected any abnormal radiation levels and Romania's watchdog said there had been no incident at the country's sole nuclear plant.
