French investigators said on Thursday they suspect that US 2 million dollars paid to a son of disgraced athletics supremo Lamine Diack was to get support for Tokyo's 2020 Olympics bid.

Some 2.8 million Singapore dollars paid to a company owned by Papa Massata Diack is at the centre of the suspicions, prosecutors said in a statement. Diack father and son already face corruption charges in France.

The Japanese government insisted earlier on Thursday that the Tokyo bid was "clean".

Two payments were made in 2013 to Black Tidings, a Singapore-based company linked to Papa Diack, who was employed by the International Association of Athletics Federations (IAAF) as a consultant, French prosecutors said in a statement.

The International Olympic Committee chose Tokyo over Istanbul and Madrid as host for the 2020 Games in September 2013. Diack senior was still an IOC member at the vote.

The payments were discovered as part of an inquiry into allegations that Diack father and son organised bribes to cover up failed dope tests by Russian athletes, the prosecutors said. French investigators became involved as the money may have been laundered in Paris.

"The National Financial Prosecution service was informed of two financial movements alleged to have been carried out in July and October 2013," said the statement.

It said the money totalled 2.8 million Singapore dollars and was "labelled as 'Tokyo 2020 Olympic Games Bid', coming from an account opened at a Japanese bank, for the profit of the 'Black Tidings' company in Singapore".

The prosecutors said the payments "so close to the International Olympic Committee's designation of the organising city for the 2020 Olympic Games, important parallel purchases by Black Tidings in Paris", and other elements had convinced them to start a new inquiry.