The fancybear.net website said the group had hacked the WADA databases and would start by releasing exclusive information on the US team followed later by other national Olympic teams.

Confidential information

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 Mr Niggli.

"We are reaching out to stakeholders... regarding the specific athletes impacted."

Mr 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.

Ms 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.

Reuters