BERLIN (AP) — The U.N. nuclear agency said Friday that slightly elevated levels of radioactivity detected in northern Europe likely were related to a nuclear reactor that was either operating or undergoing maintenance, but it's still unclear where it is located.

Estonia, Finland and Sweden last week measured higher-than-usual levels of ruthenium and caesium isotopes and detected some other artificial radionuclides. They said nothing on their territory had happened to explain their presence, as did more than 40 other countries that volunteered information to the International Atomic Energy Agency.

The IAEA said the higher level of isotopes “is likely related to a nuclear reactor that is either operating or undergoing maintenance, when very low radioactive releases can occur.” It added in a statement that “the geographical origin of the release has not yet been determined.”

The Vienna-based U.N. agency said it “ruled out that the release was related to the improper handling of a radioactive source.”

“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,” it said. The agency stressed anew that the concentrations of the particles in the air were very low and posed no risk to human health or the environment.

The Netherlands’ National Institute for Public Health and the Environment said a week ago that the isotopes may be from a source in Russia and “may indicate damage to a fuel element in a nuclear power plant.” Russia’s state nuclear power operator, however, said the two nuclear power plants in northwestern Russia haven’t reported any problems.

Russia was among the countries that reported back to the IAEA after it contacted nuclear authorities across Europe to request information.