The IAAF says it has been hacked by the 'Fancy Bears' group and that information on athletes' therapeutic use exemption (TUE) applications has been compromised.
Athletics' world governing body was notified of unauthorised remote access to its network last month but it has admitted it is unaware whether information was stolen from the network.
Athletes who have applied for TUEs, which can be issued to athletes who have an illness or condition that requires the use of medication that is on the World Anti-Doping Agency's prohibited list, were contacted on Monday.
"Our first priority is to the athletes who have provided the IAAF with information that they believed would be secure and confidential," said IAAF president Sebastian Coe.
"They have our sincerest apologies and our total commitment to continue to do everything in our power to remedy the situation and work with the world's best organisations to create as safe an environment as we can."
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