"One possible hypothesis is that of a leak coming from an incident during an operation involving radioactive fuel cooled for around two years in Mayak complex, which is in the area between the Volga and the Urals," France's Institute for Radioprotection and Nuclear Safety (IRSN) said.

European monitoring stations began detecting increased levels of the Ruthenium-106 isotope in the atmosphere in late September.

Russian authorities said they were not aware of any accident on their territory.

The IRSN had said at the time it believed the radiation came from the area between the Volga river and the Ural mountains and that it suspected a leak from a nuclear fuel treatment site rather than an accident in a nuclear reactor.

It said the radiation had not been harmful to public health or the environment.

Russia has set up an international commission of experts to investigate the matter.

The Mayak plant suffered one of the world's worst nuclear accidents in 1957 when an explosion caused radiation to be released over a wide area.

Today, the site houses a nuclear reprocessing plant.