USA Gymnastics has confirmed Simone Biles was given permission to use a banned substance at the Rio Olympic Games after the teenage sensation was among athletes whose medical data was obtained in a cyber-attack.

The World Anti-Doping Agency (WADA) on Tuesday revealed it has been hacked by a Russian cyber espionage group going by the name of Tsar Team (APT28) - also known as Fancy Bear.

Biles, a four-time gold medallist in Rio, as well as Serena and Venus Williams were some of the athletes whose information was accessed by the group.

USA Gymnastics confirmed Biles had been cleared to take medication that she has long been prescribed because she suffers from attention deficit hyperactivity disorder.

A USA Gymnastics statement said: "USA Gymnastics recently learned that the database of the World Anti-Doping Agency (WADA) has been hacked and that drug-testing reports for several U.S. Olympic athletes, including Simone Biles, have been breached.

"In keeping with official protocols of the U.S. Anti-Doping Agency and WADA, Biles submitted and was approved for a therapeutic-use exemption (TUE), the proper paperwork for any medications that an athlete takes for an illness or condition that requires the use of a medication included on the WADA Prohibited Drug List, for prescribed medication(s) she takes.

"By virtue of the TUE, Biles has not broken any drug-testing regulations, including at the Olympic Games in Rio."

Steve Penny, president of USA Gymnastics, added: "Simone has filed the proper paperwork per USADA and WADA requirements, and there is no violation.

"The International Gymnastics Federation, the United States Olympic Committee and USADA have confirmed this. Simone and everyone at USA Gymnastics believe in the importance of a level playing field for all athletes."

Biles tweeted: "I have ADHD and I have taken medicine for it since I was a kid. Please know, I believe in clean sport, have always followed the rules, and will continue to do so as fair play is critical to sport and is very important to me.

"Having AHDH, and taking medicine for it is nothing to be ashamed of [and] nothing that I'm afraid to let people know."

United States Anti-Doping Agency chief executive Travis T. Tygart said: "It's unthinkable that in the Olympic movement, hackers would illegally obtain confidential medical information in an attempt to smear athletes to make it look as if they have done something wrong.

"The athletes haven't. In fact, in each of the situations, the athlete has done everything right in adhering to the global rules for obtaining permission to use a needed medication.

"The respective International Federations, through the proper process, granted the permission and it was recognized by the IOC and USADA.

"The cyber-bullying of innocent athletes being engaged in by these hackers is cowardly and despicable. It is time for the entire international community to stand up and condemn this cyber-attack on clean sport and athlete's rights."