By Karolos Grohmann

RIO DE JANEIRO (Reuters) - Doping cheats using designer drugs or gene doping at the Rio Olympics should know they will eventually be found out, the International Olympic Committee said on Monday.

The Rio Games are taking place under the cloud of doping with the Russian track and field team virtually excluded after a state-backed systematic doping program was revealed, and three more athletes having tested positive since the opening of the Games village on July 24.

But that may not be the end of the story and about 6,000 tests, both urine and blood, will be stored by the IOC for a decade to allow for re-testing, using new technology, and the detection of substances that were not yet known at the time.

"We will store the samples. We can be very confident that an athlete who is cheating should be very scared," IOC medical chief Richard Budgett told reporters.

He said athletes guilty of gene doping or using the so-called designer drugs -- more sophisticated forms of performance-enhancing drugs -- will be caught.

"If someone thinks they have designer drugs eventually they will be found," he said. "The message for all those cheats out there is 'beware you will be caught.'

"I am confident we have the deterrents that should lead to the protection of clean athletes."

Targeted re-tests from the Beijing 2008 and the London 2012 Games recently yielded an additional 98 positive samples for banned substances with none of those athletes able to compete in Rio.

A test for gene doping would also be used in a matter of months after the World Anti-Doping Agency (WADA) approved it recently.

Budgett did not want to say whether the Rio Games would be drugs-free but he said he was confident over time the cheats would be discovered.

"I am a medical man fundamentally so we always doubt everything. That is our job. So I think it is right we always investigate. But it is unfortunate that we doubt every fantastic performance. That's why we test the top five athletes. You can never know."

(Reporting by Karolos Grohmann, editing by Neil Robinson)