MOSCOW — Russia said Tuesday for the first time that it had detected a significant radiation spike in the Ural Mountains, close to a sprawling Soviet-era nuclear plant still remembered as the site of an accident 60 years ago. But it rejected suggestions that it was the source of a radioactive cloud that hovered over Europe.

The location of the spike — in the Chelyabinsk region near the border with Kazakhstan — has been identified by French and German nuclear safety institutions as a potential source for a concentration of a radioactive isotope called ruthenium 106 detected in the air in late September above several European countries.

But nuclear energy authorities in Moscow insisted Monday that still-higher levels of atmospheric contamination had been detected outside Russia, in southeastern Europe.

Reports of the elevated radiation levels over Western Europe raised alarms, but nuclear safety authorities in France and Germany said there was no threat to human health or to the environment — an assurance repeated on Tuesday by Moscow.