Russian hackers have broken into a World Anti Doping Agency database and posted confidential information online about American athletes and have threatened of more leaks to come.
WADA identified the group of hackers as Tsar Team (APT28), also known as Fancy Bear.
The www.fancybear.net website said it had information about a number of U.S. athletes, including tennis sisters Serena and Venus Williams as well as multiple gold medal-winning gymnast Simone Biles.
"WADA condemns these ongoing cyber-attacks that are being carried out in an attempt to undermine WADA and the global anti-doping system," said director general Olivier Niggli in a statement that did not name any athletes.
"WADA has been informed by law enforcement authorities that these attacks are originating out of Russia."
Russian news agencies quoted Kremlin spokesman Dmitry Peskov as saying any possible Russian government or secret service participation in the hacking was "out of question".
Russia's track and field team, with the exception of one athlete based in the United States, were banned from the Rio Games in August over what WADA said was a state-backed doping programme.
Russian competitors in other sports also had to prove they were clean by meeting several criteria to be eligible to compete in Brazil.
The fancybear.net website said the group had hacked the WADA databases and would start by releasing exclusive information on the U.S. team followed later by other national Olympic teams.
WADA said the group was believed to have gained access to its anti-doping administration and management system (ADAMS) via an International Olympic Committee-created account for the Rio Games.
Data accessed included so-called Therapeutic Use Exemptions (TUE) issued by sports federations and national anti-doping organisations that allow athletes to take certain substances.
"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," said Niggli.
"We are reaching out to stakeholders... regarding the specific athletes impacted."
Niggli added that the hack was "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."
The McLaren report described, among other things, how Russians were replacing positive doping samples with clean ones during the Sochi winter Games with the support of the Russian secret service.
WADA revealed last month that Russian whistleblower Yulia Stepanova's electronic account had been illegally accessed with a "perpetrator" obtaining details which would normally include her registered whereabouts.
Stepanova, in hiding in North America, helped reveal the biggest state-backed doping programme in Russia and was forced to flee the country with her husband for fear of her life.