French prosecutors are investigating the decision by global soccer body FIFA to award the 2018 World Cup to Russia and the 2022 tournament to Qatar, a source close to the investigation said on Thursday.

The national financial prosecutor's office (PNF), which specialises in investigating economic crimes, began a preliminary investigation last year.

It is looking into possible offences which include private corruption, conspiracy and influence peddling, the source said.

The source added that former FIFA president Sepp Blatter was interviewed as a witness by Swiss prosecutors in the presence of two French magistrates in Switzerland on April 20.

The 81-year-old Blatter, who has been banned for six years by FIFA's own ethics committee for ethics violations, was not immediately available to comment.

FIFA has been in turmoil since several dozen sports officials, including a number of senior FIFA officials, were indicted in the United States on corruption-related charges in 2015.

In the same year, Swiss authorities opened criminal proceedings against individuals on suspicion of mismanagement and money laundering related to the allocation of the 2018 and 2022 tournaments.

Hosting rights for both tournaments were awarded at the same vote by the FIFA executive committee in Zurich in December 2010.

Two members of the committee, Reynald Temarii and Amos Adamu, were suspended one month before the vote after they allegedly offered to sell their votes to Sunday Times reporters posing as lobbyists for an American consortium.

As a result, only 22 of the 24 executive committee members voted.

Russia and Qatar have denied any wrongdoing and FIFA's own investigation, completed in 2014, said it had found no grounds to strip either country of the right to host the event.