LOS ANGELES • The World Anti-Doping Agency (Wada) has slammed Russian hackers who breached its database and published confidential records of American Olympic gymnast Simone Biles and tennis stars Venus and Serena Williams.

The agency said in a statement that the Russian cyber-espionage group Tsar Team (APT28), also known as Fancy Bears, had broken into its Anti-Doping Administration and Management System (Adams) database.

The hacking group released information gleaned from the files of Biles, the Williams sisters and US women's basketballer Elena Delle Donne.

In a posting on its website, Fancy Bears claimed US athletes at the Olympics had "played well but not fair". However, none of the documents published by the group provided evidence of wrongdoing on the part of the athletes involved.

Instead, the disclosed files set out instances where the athletes had been granted exemptions to use various medications for legitimate reasons - a common practice in the sports world.

The United States Anti-Doping Agency (Usada) characterised the hack as a "cowardly and despicable" attempt to smear the four women.

"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," Usada chief executive Travis Tygart, who famously helped expose US cyclist and dope cheat Lance Armstrong, said.

Biles said on Twitter that she had taken medication for attention deficit hyperactivity disorder (ADHD) since childhood.

"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," said the 19-year-old, who won four golds at the Rio Olympics.

Venus expressed "disappointment" at the breach, stating: "I am one of the strongest supporters of maintaining the highest level of integrity in competitive sport."

Delle Donne laughed off the revelations in a upbeat post on Twitter. "I'd like to thank the hackers for making the world aware that I legally take a prescription for a condition that I've been diagnosed with, which Wada granted me an exemption for. Thanks guys!"

An International Olympic Committee (IOC) spokesman said the leaked information was "clearly aimed at tarnishing the reputation of clean athletes" while reiterating that no violation had occurred.

The data breach comes just weeks after hackers gained access to Wada's file on Russian doping whistle-blower Yulia Stepanova.

Stepanova, who is hiding in the United States, later said she feared for her life following the hack.

Wada said it believed the latest breach occurred after "spear phishing" of e-mail accounts and that it had been confined to Adams accounts of athletes competing in Rio. Spear phishing is when an e-mail user receives a message purportedly from someone they know, but it is actually from a hacker.

The hacking comes after a series of Wada investigations which have alleged a vast state-sponsored doping programme in Russian sport dating back several years.

Wada director-general Olivier Niggli said the hacking would hamper Russian efforts to reintegrate into the sports world.

But Dmitry Peskov, spokesman for President Vladimir Putin, said the Russian government was not involved in the hacks.

Fancy Bears, however, indicated it planned to release more information. "This is just the tip of the iceberg," the group said on its website.

"Wait for sensational proof of famous athletes taking doping substances any time soon."

AGENCE FRANCE-PRESSE, NYTIMES