Russia's weather service acknowledged it had measured pollution of ruthenium-106 at 1,000 times normal levels in the Ural Mountains.
It said there was no health risk.
The announcement appeared to confirm a report by France's nuclear safety institute which detected a cloud of radioactive pollution over Europe.
The Institute for Radioprotection and Nuclear Safety (IRSN) said on 9 November it had detected ruthenium-106 in France. It added that the source of contamination could have been an accident at a nuclear facility in either Russia or Kazakhstan.
