The World Anti-Doping Agency says Russian hackers broke into its database and posted some athletes' confidential medical data online.

WADA says the attack - which targeted some American female athletes who competed at the Rio de Janeiro Olympics - was carried out by a "Russian cyber espionage group" called Fancy Bears.

The hackers revealed records of "Therapeutic Use Exemptions" which allow athletes to use substances that are banned unless there is a verified medical need.

WADA previously warned of cyberattacks after investigators it appointed published investigations into Russian state-sponsored doping.

In a statement, WADA director general Olivier Niggli says "these criminal acts are greatly compromising the effort by the global anti-doping community to re-establish trust in Russia."

WDA says it has "extended its investigation with the relevant law enforcement authorities."