French swimmer Camille Lacourt has waded into the Olympic doping controversy, casting doubt on new 200m freestyle champion Sun Yang.

Lacourt, who came fifth in the 100m backstroke final just after Sun's triumph, said swimming was becoming like athletics "with two or three doped in each final".

The Rio swimming contest has become the new centre of the international doping storm unleashed by a report for the World Anti-Doping Agency, which found the Russian Government organised the use of banned substances and manipulated doping tests.

Sun, who served a three-month ban in 2014, has been caught in a doping controversy after Australian 400m gold medallist Mack Horton labelled him a drug cheat, which kicked off a furore between the two countries' teams.

Sun secretly served a three-month ban after testing positive for trimetazidine, which Chinese officials said he had taken for years to treat an existing heart problem before the stimulant was added to World Anti-Doping Agency's banned list in January 2014.

And now Lacourt, who finished 0.27 of a second behind Australia's Mitch Larkin in the 100m backstroke final, has criticised the Chinese star.

"Sun Yang, he pisses purple," Lacourt told French radio station RMCsport after his race.

"When I see the 200m podium I want to be sick. I prefer to remember the crowd that cheered when we went out.

"I am very sad when I see my sport getting like this. I have the impression I am looking at athletics, with two or three doped in each final.

"I hope that [swimming's world governing body] FINA is going to react and stop this massacre, because it is getting sad."

'I want to be sick' ... China's Sun Yang powered to victory in the 200m freestyle. ( AP: Matt Slocum )

It is just the latest salvo in a strong push against convicted dopers that has become an underlying theme of the Rio Games.

Apart from the criticism of Sun from Horton and Lacourt, American Lilly King continued her ongoing rivalry with Russian Yulia Efimova.

Efimova had a doping ban overturned at the 11th hour in order to compete in Rio, and 19-year-old King called her out after edging the 100m breaststroke final.

Michael Phelps, who will swim for his 20th Olympic gold medal on Wednesday (AEST), backed his young team-mate, saying "something needs to be done" to prevent doping from infiltrating swimming.

AFP/ABC