France has found traces of radioactive caesium on mushrooms imported from Russia, the head of French nuclear regulator ASN says.

ASN chief Pierre-Franck Chevet told the French senate that following the discovery of a cloud of radioactive pollution coming from Russia in October, levels of radioactive ruthenium 106 in the air had posed no safety threat to French citizens, but controls on food imports had been strengthened.

"The latest information I have is that it seems that traces of caesium have been found on mushrooms that would have come from Russia," Mr Chevet told the Senate hearing on nuclear security.

Officials at French consumer protection agency DGCCRF were not immediately available for comment and the ASN did not immediately respond to request for further detail.

In recent weeks, France has imported chanterelle mushrooms from Russia which are on sale in supermarkets in Paris.

Mr Chevet said the caesium traces did not appear to be in line with the calculations by French nuclear safety institute IRSN, which on November 9 said it had detected unusual amounts of only ruthenium 106 in the atmosphere.

"There is a contradiction between what has been measured and what has been calculated by the IRSN. Work is continuing," Mr Chevet said.

This Institute for Radiological Protection and Nuclear Safety map shows the detection of Ruthenium-106 in Europe. ( AP: INRS )

Scientists have said the presence of ruthenium without other radioactive elements would indicate there had probably been a spill of ruthenium rather than a bigger nuclear accident.

Russian state weather service Roshydromet said last week it had found "extremely high pollution" of ruthenium 106 at nearly 1,000 times normal levels near the Mayak nuclear fuel reprocessing plant in southern Russia, owned by Russian state nuclear company Rosatom.

Mayak has denied that its plant was the source of the increased level of ruthenium 106.

Rosatom said there were no accidents at any of its facilities which could increase the level of ruthenium 106 in the atmosphere.

Reuters