Hacktivists have released files claiming that top US athletes received a green light from WADA to take banned substances. The US anti-doping agency, sports federations and athletes themselves have gone public to deny any wrongdoing.

The allegations were published on the website of the hacktivist group Fancy Bear, which described the revelations as being “just the tip of the iceberg.”

The World Anti-Doping Agency (WADA) has released a statement confirming and condemning the cyber attack. It noted that the cyber intrusion was conducted by a Russian hacker team and that it is taking the attack “very seriously.”

WADA Confirms Attack by Russian Cyber Espionage Group: https://t.co/PTBIxeOM9w — WADA (@wada_ama) September 13, 2016

“The group accessed athlete data, including confidential medical data - such as Therapeutic Use Exemptions delivered by International Sports Federations (IFs) and National Anti-Doping Organizations (NADOs) - related to the Rio Games.”

According to the body, the information was obtained “through spear phishing of email accounts.”

“WADA condemns these ongoing cyber-attacks that are being carried out in an attempt to undermine WADA and the global anti-doping system,” said Olivier Niggli, WADA director general.

Kremlin spokesman Dmitry Peskov has strongly denied any possible Russian involvement in the hacking attack on WADA.

READ MORE: Kremlin denies Russian involvement in WADA hack

The United States Anti-Doping Agency (USADA) has also issued a response, saying that “the athletes have done nothing wrong.”

“In fact, in each of the situations, the athlete has done everything right in adhering to the global rules of obtaining permission to use a needed medication,” USADA CEO Travis T. Tygart stated.

He added that all the medications were approved by the respective international federations, WADA and the International Olympic Committee (IOC).

Statement from USADA CEO, Travis T. Tygart, on Russian cyber-espionage grouphttps://t.co/UTkqPY3VNIpic.twitter.com/ZPWNQCPtDz — USADA (@usantidoping) September 13, 2016

The documents the group says it hacked from WADA’s Anti-Doping Administration and Management System (ADAMS) database contain information that Serena Williams, one of the world’s greatest ever tennis players, was taking a number of banned substances.

Williams was allowed to take oxycodone, hydromorphone, prednisone and methylprednisolone in 2010, 2014 and 2015, despite the substances being placed on WADA’s list of banned substances.

However, the documents released also showed that Williams had been given special permission to take some of the drugs. The authorization was given by Dr. Stuart Miller from the International Tennis Federation (ITF).

Serena’s older sister Venus was found to have taken prednisone, prednisolone, triamcinolone and formoterol, which are also on WADA’s banned list. However, she was given special dispensation to take the substances as long as she adhered to various conditions regarding the use of the drugs.

Both Serena and Venus Williams were allowed to take the prohibited drugs, the ITF says.

"The Williams sisters had permission for the therapeutic use of banned drugs," the ITF is quoted as saying by TASS.

Meanwhile, American gymnast Simone Biles tested positive for methylphenidate after giving a urine sample. However, like the Williams sisters, she was given special dispensation to take the drug.

RT has contacted the International Federation of Gymnastics for comment, with its representative saying the body followed the current rules and that there is “no problem.”

“She [Biles] has a therapeutic use exemption, there is nothing we have to mention about that, everything is absolutely clear and according to the rules.”

Reacting to the leak, Simone Biles issued a statement on Twitter, saying that she has “nothing to be ashamed of,” since she suffers from attention deficit hyperactivity disorder (ADHD).

“I have ADHD and I have taken medicine for it since I was kid. Please know, I believe in clean sport, have always followed the rules, and will continue to do so,” Biles stressed, adding that sporting ethics are very important to her.

According to the leaked papers, US basketball star Elena Delle Donne's drug test revealed that she had used amphetamine.

The athlete also reached out to the public on social media “thanking” the hackers for their move.

“I’d like to thank the hackers for making the world aware that I legally take a prescription for a condition I’ve been diagnosed with, which WADA granted me an exemption for,” Donne wrote.

The revelations also pointed out that since 2014 the athlete has been taking hydrocortisone, which falls into the glucocorticoids bracket (banned by WADA), which is classified as doping.

The leaked documents do not contain any diagnoses for the affected athletes, with the respective fields in the papers marked as "confidential."