The statement appeared just hours after the Russia's meteorological service (Rosgidromet) reported that it has detected the record levels of radiation in Russia's Ural regions and a radioactive cloud that drifted over Europe in early October.
The level of ruthenium-106, a radioactive isotope that does not occur naturally, had reached 986 times the normal level of pollution at a monitoring station about 20 miles from the Mayak nuclear facility, where in 1957 an explosion exposed at least 272,000 people to risky levels of radiation.
The data corroborated conclusions reached by France's nuclear regulator, which reported 11 days ago that a cloud of radioactive pollution detected over Europe likely came from an accident that could have taken place in Russian Federation or Kazakhstan during the last week of September.
France's IRSN, a nuclear safety institute, ruled out the possibility of an accident in a nuclear reactor and believes the material it detected is more likely to have been released from a nuclear fuel treatment site or centre for radioactive medicine.
Ruthenium-106 is a radioactive isotope that is a by-product of a nuclear reaction and it is not found in nature.
A radioactive cloud that engulfed Europe earlier this year originated in Russian Federation, researchers say.
Russia's meteorological agency, Rosgidromet, said pollution was of Ru-106 almost 1,000 times normal levels at the Argayash weather station in late September and early October.
It said there was no health risk.
The Mayak plant was the scene of a major nuclear accident in 1957, when a waste storage facility blew up.
Their findings suggest the radiation could have come from a medical research plant, a fallen satellite or a nuclear fuel reprocessing site.
Ruthenium-106 is a product of splitting atoms in a reactor and does not occur naturally.
But the fact that Roshydromet itself detected the radiation cloud using two monitoring stations surrounding the Mayak nuclear facility - one of the largest plants in Russian Federation - is leading others to question those claims.
The exact source of the spike, however, remains unknown, though IRSN suggested that the cause might be an accident.
Roshydromet adds a large anticyclone in the White Sea played a key role in ferrying the radioactive particles to parts of western Europe, where they were detected by European monitors.
"Greenpeace will send a letter asking prosecutors to open an inquiry into potential concealment of a nuclear incident", the charity said in a statement.