The head of world athletics admits the sport faces a crisis over allegations of doping by Russian competitors.
However, the president of the International Association of Athletics Federations, Lamine Diack, dismissed suggestions that his organisation covered up failed drugs tests.
Diack says allegations that 99 percent of Russian athletes use performance enhancing drugs are a joke.
In his first broadcast interview since the allegations were made public, Diack said he was "shocked" and "disturbed" when he first heard the claims.
But the 81-year-old Senegalese official insists allegations that 99% of Russian athletes are doping were "a joke" and "ridiculous".
The former IAAF president Lamine Diack. Photo: Supplied
"I cannot accept that somebody came and said in Russia it's 99% cheating. It's not true," he says. "I understand after this kind of crisis people are saying 'OK, what are they doing, is it right or not?', but I think we have to be absolutely clear that our athletes are 90% to 95% clean."
A series of documentaries by German TV channel WRD has presented evidence of alleged systematic doping and corruption in Russian sport
The allegations include:
Russian Anti-Doping Agency (Rusada) officials supplied banned substances in exchange for 5% of athletes' earnings and colluded with doping control officers to hush up and falsify tests.
IAAF officials are implicated in covering up Russian doping - including Diack's son, IAAF marketing consultant Massata Papa Diack, Valentin Balakhnichev, the IAAF treasurer and Russian Athletics Federation president, and IAAF legal adviser Habib Cisse.
The IAAF decided not to look into 150 suspicious blood samples between 2001 and 2008, including an unnamed top British athlete.
German television station WDR broadcast three documentaries alleging that IAAF officials were implicated in covering up doping in Russia.
Those claims are now being examined by the IAAF's ethics commission, which will decide whether Russian anti-doping officials, its own treasurer and even the son of its president are guilty of wrongdoing.
But Diack - who will stand down as IAAF president in August after 16 years as the most powerful figure in track and field - denied knowledge of any cover-up.
"I'm convinced I know my department. I know how they work very, very hard about the fight against doping, and I didn't see any reason to make a cover-up of a doping case," he says.
Athletics has a chequered history of drug scandals, from East Germany's years of state-imposed blanket doping, through to the Bay Area Laboratory Co-Operative (Balco) affair. However, it was one of the first sports to introduce the biological passport.
These latest claims of widespread wrongdoing stem principally from former Russian Anti-Doping Agency (Rusada) official Vitaly Stepanov and his wife Yulia (nee Rusanova), formerly an 800m runner who was banned for doping.
They allege that leading Russian athletics officials supplied banned substances in exchange for 5% of an athlete's earnings and colluded with doping control officers to hush up and falsify tests.