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Hackers have released a number of stolen medical files belonging to Olympic athletes.

The World Anti-Doping Agency (WADA) confirmed that its website had been attacked by a Russian "cyber espionage group", but it did not say when the attack was carried out.

A group calling itself Fancy Bears took responsibility for the hack. The hacker group first appeared on Twitter over a week ago, but it is also known as APT28, an "advanced persistent threat" group, which security firm FireEye has previously linked to the Russian government.

The leaked documents include internal medical files of tennis players Serena and Venus Williams, Olympic gymnast Simone Biles, and basketball player Elena Delle Donne.

WADA said in a statement that the attack was "carried out in an attempt to undermine WADA and the global anti-doping system".

The agency said that the hackers used a spearphishing attack on email accounts, but it added that the leaked data is "confined" to data held on athletes who competed in the Rio Olympics earlier this year.

The hackers indicated that more documents would soon be released.

"This is just the tip of the iceberg," said the hacker group on its website, which we are not linking to.

We reached out to the hacker group by email but did not hear back at the time of writing. An email to WADA was not immediately responded to.