The World Anti-Doping Agency has hit out at Russian hackers for leaking confidential medical files of a number of the United States' Olympic athletes.

Superstar tennis pair Serena and Venus Williams are among those to have been affected by the data breach, carried out by a group referring to itself as "Fancy Bears".

Teenage gymnast Simone Biles, who won four gold medals at Rio 2016, was accused of taking an "illicit psycho-stimulant", which she has since claimed was purely to treat her Attention Deficit Hyperactivity Disorder.

In a statement released on Tuesday evening, WADA condemned the hackers in question and claimed that it was possibly an attack to undermine the global anti-doping system.

"WADA deeply regrets this situation and is very conscious of the threat that it represents to athletes whose confidential information has been divulged through this criminal act," it read. "We are reaching out to stakeholders, such as the IOC, IFs and NADOs, regarding the specific athletes impacted.

"WADA condemns these ongoing cyber-attacks that are being carried out in an attempt to undermine WADA and the global anti-doping system. WADA has been informed by law enforcement authorities that these attacks are originating out of Russia.

"Let it be known that these criminal acts are greatly compromising the effort by the global anti-doping community to re-establish trust in Russia further to the outcomes of the Agency's independent McLaren Investigation Report."

A damning WADA report into state-sponsored cheating in the Russian Federation camp - which the Russian government has since denied any involvement in - led to many athletes being banned from competing in Rio, while a blanket ban was handed to all those hoping to take part in the Paralympics.