The International Atomic Energy Agency (IAEA) said it could not yet determine where the reactor was located after several northern European countries, including Finland and Sweden, noted elevated levels of radioactivity last week.

One Dutch institute said the source was western Russia, while another theory was that it came from wood burned in Latvia that was still contaminated from the Chernobyl nuclear disaster.

Russian nuclear power producer Rosenergoatom has denied any incident at its two plants, while a French NGO dismissed the Latvia theory based on the types of isotopes that were detected.

The raised levels were "likely related to a nuclear reactor that is either operating or undergoing maintenance, when very low radioactive releases can occur", the IAEA said in a statement.

The Vienna-based organisation said more than 40 countries, including Russia, reported nothing unusual on their soil.

"The IAEA ruled out that the release was related to the improper handling of a radioactive source," it said.

"It was also unlikely to be linked to a nuclear fuel processing plant, a spent fuel pool or to the use of radiation in industry or medicine."

The agency said that based on data reported by its member states, the observed air concentrations of the particles "were very low and posed no risk to human health and the environment".