A Russian hacking group has alleged that Serena and Venus Williams and Simone Biles were allowed to consume banned substances by WADA after claiming to have accessed its database.

Moscow: A Russian hacking group has alleged that American tennis stars Serena and Venus Williams and Rio Olympics gold medallist gymnast Simone Biles were allowed to consume banned substances by World Anti-Doping Agency (WADA) after claiming to have accessed WADA's database.

Fancy Bears' hack team claimed to have leaked dozens of files allegedly relating to United States athletes on the internet, according to media reports.

"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. The US team played well but not fair."

According to the Fancy Bears' group, Serena was allowed to take banned substances such as oxycodone, hydromorphone, prednisone and methylprednisolone in 2010, 2014 and 2015. Venus was allowed to take prednisone, prednisolone and triamcinolone among others in 2010, 2011, 2012 and 2013.

Biles, who won four gold medals in the 2016 Olympics in Rio, tested positive for methylphenidate in August, but she was not disqualified. In 2013 and 2014, she was allowed to take dextroamphetamine, the group stated.

Meanwhile, WADA's director general Olivier Niggli was quoted as saying that hackers have been trying to break into agency's website on a daily basis over the past three weeks.