Low levels of radioactive particles have been detected in the Czech Republic and elsewhere in Europe, the UN nuclear agency has said.
The iodine-131 particles do not pose a public health risk, the International Atomic Energy Agency said.
The body said it was trying to work out where the particles had come from but said it did not believe the source was Japan's stricken Fukushima plant.
It said the Czech Republic had first informed them of the raised levels.
"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," the body said in a statement.
It said iodine-131 was a short-lived radioisotope with a radioactive decay half-life of about eight days.
The Czech nuclear security authority said it had been detecting radioactive iodine-131 at a number of monitoring stations since late October and had informed the IAEA to see if it could identify the source, Reuters reports.
Czech nuclear safety chief Dana Drabova said the iodine could have leaked during production of radiopharmaceuticals.
It was certainly not from a nuclear power plant, she said, adding that they were almost certain that the source was abroad.
Slightly elevated readings of radioactive iodine have also been found in northern Germany, the country's environment ministry said.
But the levels were so low they were barely detected, a spokeswoman quoted by Reuters news agency said.
The environment ministry spokeswoman said the radioactivity could not have come from a nuclear power plant.
In the days and weeks after the Japan's accident, minuscule amounts of iodine-131 believed to have come from Fukushima were detected as far away as Iceland and other parts of Europe, as well as in the US.