Frascati - Two professional riders, Slovenian Kristijan Koren and Croatian Kristijan Durasek have been suspended as police investigate an Austrian blood doping ring, cycling's world governing body the UCI said on Wednesday.

Two other retired riders, Italian Alessandro Petacchi and Slovenian Borut Bozic, have also been implicated in the Operation Aderlass investigation.

Koren, 32, who rides for the Bahrain team of Italian star Vincenzo Nibali, has been competing in the Giro d'Italia, which started this week and was 47th overall after Tuesday's fourth stage.

UAE Emirates' Durasek was riding in the Tour of California.

According to Italian sports daily Gazzetta dello Sport, who organise the three-week Giro, sources "close to the dossier" claim that Koren and Bozic's implication dates back to 2012-2013.

Koren was riding for the Cannondale team at the time, while Bozic, whose 21 wins include a stage of the 2009 Tour of Spain, rode for Astana.

Durasek, 31, who won the Tour of Turkey in 2015, is alleged to have more recent links.

Petacchi, who has won 48 Grand Tour stages, including 22 on the Giro d'Italia, has also been implicated, according to media reports on Tuesday.

Petacchi, who retired in 2015 won six Tour de France stages and 22 on the Giro, has denied the allegations outright.

"I never had a blood transfusion and I have no idea why I'm being connected with this case," the 45-year-old told Corriere della Sera.

Bozic retired last year and joined the Bahrain team management, but was not present on the Giro.

Operation Aderlass concerns German doctor Mark Schmidt, who was arrested during the Nordic ski world championships in February in a blood doping scandal involving sports stars from 21 disciplines and eight nationalities.

Blood doping is a method whereby athletes have blood transfusions, to increase their stamina and performance, a method prohibited under World Anti-Doping Agency regulations.