Moscow: Russian hacking group Fancy Bears' Hack Team on Tuesday claimed to have accessed the World Anti-Doping Agency (WADA) database, alleging that American tennis stars Serena and Venus Williams and four-time Rio Olympics gold medallist gymnast Simone Biles have been allowed to consume banned substances.

The hacking site claimed to have leaked dozens of files allegedly relating to United States athletes on the internet, reports Sputnik.

The site claimed former No.1 Serena was allowed to take banned substances such as oxycodone, hydromorphone, prednisone and methylprednisolone in 2010, 2014 and 2015.

The hacking site also claimed that Simone Biles failed a doping test yet avoided disqualification.

Serena's sister Venus was allowed to take prednisone, prednisolone and triamcinolone among others in 2010, 2011, 2012 and 2013. The documents do not reveal the diagnoses of the athletes which led to WADA allowing them to take the substances.

Simone Biles tested positive for methylphenidate in August, but she was not disqualified. In 2013 and 2014, she was allowed to take dextroamphetamine.

Simone Biles, also a ten-time world championships gold medallist, tested positive in August 2016.

"After detailed studying of the hacked WADA databases we figured out that dozens of American athletes had tested positive," the Fancy Bears' group claimed on its website.

"The Rio Olympic medallists regularly used illicit strong drugs justified by certificates of approval for therapeutic use," it said.

"In other words they just got their licenses for doping. This is other evidence that WADA and IOC's Medical and Scientific Department are corrupt and deceitful."

"As predicted, the USA dominated the 2016 Olympics medal count with 46 gold, 37 silver, 38 bronze for 121 total," Fancy Bears' continued.

"The US team played well but not fair."

The site also claimed American basketball star Elena Delle Donne's drug test revealed that she had also used amphetamine. In addition, since 2014 she has been taking hydrocortisone that is also classified as doping.

The entire Russian athletics team was barred from competing in Rio after systemic doping breaches were exposed earlier this year.

Since then, several hacking groups have threatened to hack WADA.