Professional cyclists Kristijan Koren of Slovenia 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 in spotlight -

Petacchi, who won 48 Grand Tour stages including 22 on the Giro before retiring in 2015, has been cited by the UCI for a potential "use of prohibited methods".

The 45-year-old has also been temporarily dropped from his role as a television pundit on this year's Giro with Italian public broadcaster RAI.

"For us, nothing changes," said Raisport director Auro Bulbarelli.

"But, faced with these accusations, it is right that Alessandro has the opportunity to assess the situation with his lawyer.

"We believe it is appropriate to take a few days off to clarify all the details of this story."

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

The Bahrain team, which has suspended Koren and Bozic, stressed that "the two cases concern the seasons of 2012 and/or 2013".

"The team is still conducting extensive medical checks (including a biological passport check) of any new riders signed," the team added.

The UCI said that both they and the Cycling Anti-Doping Foundation (CADF) "have been in close contact with the sport and state authorities involved in the Aderlass investigation, in particular with the World Anti-Doping Agency (WADA) and the Austrian law enforcement authorities".

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.