The FIS said Estonian athletes Karel Tammjärv and Andreas Veerpalu, and Kazakhstan's Alexey Poltoranin, had been handed four-year suspensions in connection with Operation Aderlass.

Estonian coach Mati Alaver has also been banned.

Disciplinary proceedings against athlete Algo Kärp and coach Andrus Veerpalu - both provisionally suspended - are still pending, the FIS added.

The four are the latest to be sanctioned as part of an Austrian police investigation which uncovered a blood doping ring and has led to the arrests of athletes from sports including skiing and cycling for their alleged involvement.

The operation was launched after Johannes Dürr made revelations about blood doping in an ARD documentary, which sparked the raid at the World Championships in February and a separate raid in Erfurt in Germany.

Authorities suspect German doctor Mark Schmidt of running a blood-doping operation involving athletes across several sports.

Poltoranin won two bronze medals at the 2013 World Championships, while Tammjärv represented Estonia at two Winter Olympic Games and Veerpalu competed at the 2010 and 2014 Winter Olympics.

Alaver, who denies wrongdoing, was given a one-year suspended prison sentence and 18 months probation by the Harju County Court in Tallinn in November.

"All have been sanctioned with a four-year suspension based on FIS anti-doping riles, article 2.2, use of a prohibited substance/method, namely blood doping and article 2.9, complicity," the FIS said.

"The sanctions are subject to appeal to the Court of Arbitration for Sport within 21 days."

At least 21 athletes from eight countries and five sports - three winter and two summer - are suspected of being involved in the worldwide blood-doping network.

Dürr, suspended for two years after being sent home from Sochi 2014 for doping, is among the other athletes to be sanctioned and was given a life ban in October.