A controversial fertility doctor claims to have cloned human embryos and implanted them into four women's wombs.

While none of the embryos Dr. Panayiotis Zavos claims to have cloned resulted in a pregnancy, he said he'll continue trying.

Cloning human embryos is not new; others have done so as part of stem cell research. But Zavos is the first to have claimed to have tried implanting them into women.

Zavos said he cloned 14 embryos, but implanted only 11. He says he believes that the women volunteers -- three who were married and one single -- were probably not "ideal subjects," which is why they all failed to become pregnant.

The controversial doctor also claims he has created human-animal "hybrid" clones by fusing the cells of dead people with the empty egg cells of cows. These hybrid embryos were created to study the cloning process, and weren't used for embryo transfer, Zavos said.

The procedures were recorded for a documentary that aired Wednesday on the Discovery Channel in Britain. But other fertility experts say Zavos has not produced any scientific evidence to support his statements, which they say can only be done by publishing the work in a peer-reviewed scientific journal.

"For his claims to have credibility, and to prevent the unethical exploitation of grieving or desperate couples Dr Zavos must throw open his work to peer review. He must demonstrate openness and allow scrutiny by experts, not just by the media," Dr. Alastair Kent, director of the Genetic Interest Group, a British charity aimed at helping families affected with inherited disorders.

"If he is as good as he claims, then he has nothing to fear. If he is not, then vulnerable women and couples need protection from his activities."

Ethicists say that Zaros is playing on the false hope of those who have lost loved ones, or are unable to conceive.

"Dr. Zavos has a lot of clients who want to be cloned or have their deceased offspring cloned . . . and they are willing to pay a lot of money to do it," David Prentice of the U.S. Family Research Council said.

Dr. Brendan Curran, a geneticist, adds: "I find it incredible and disgusting because he's sowing false hope."

Zavos says that he has tried to publish his work but that publications have rejected his manuscripts because of editorial policies not to publish what they say are illegal and unethical attempts at human reproductive cloning.

In order to evade the U.S. ban on human cloning, it's believed Zavos carried out his secret cloned embryo procedures somewhere in the Middle East as far back as 2003, Agence France Presse reported.

While Zavos was unsuccessful in achieving a viable pregnancy, he told The Independent newspaper he believed that this was just the "first chapter" in his ongoing attempts at producing a cloned baby.

"There is absolutely no doubt about it, and I may not be the one that does it, but the cloned child is coming. There is absolutely no way that it will not happen," Zavos said.