Nobel laureate Shinya Yamanaka warned patients on Tuesday about unproven "stem cell therapies" offered at clinics and hospitals in a growing number of countries, saying they were highly risky.

The Internet is full of advertisements touting stem cell cures for just about any disease -- from diabetes, multiple sclerosis, arthritis, eye problems, Alzheimer's and Parkinson's to spinal cord injuries -- in countries such as China, Mexico, India, Turkey and Russia.

Yamanaka, who shared the Nobel Prize for Medicine on Monday with John Gurdon of the Gurdon Institute in Cambridge, Britain, called for caution.

"This type of practice is an enormous problem, it is a threat. Many so-called stem cell therapies are being conducted without any data using animals, preclinical safety checks," said Yamanaka of Kyoto University in Japan.

"Patients should understand that if there are no preclinical data in the efficiency and safety of the procedure that he or she is undergoing ... it could be very dangerous," he told Reuters in a telephone interview.

Yamanaka and Gurdon shared the Nobel Prize for the discovery that adult cells can be transformed back into embryo-like stem cells that may one day regrow tissue in damaged brains, hearts or other organs.

"I hope patients and lay people can understand there are two kinds of stem cell therapies. One is what we are trying to establish. It is solely based on scientific data. We have been conducting preclinical work, experiments with animals, like rats and monkeys," Yamanaka said.

"Only when we confirm the safety and effectiveness of stem cell therapies with animals will we initiate clinical trials using a small number of patients."

Yamanaka, who calls the master stem cells he created "induced pluripotent stem cells" (iPS), hopes to see the first clinical trials soon.

"There is much promising research going on," he said.