Scientists at a U.S.-based private research firm say they have developed five human embryos using cloning technology.

Researchers from the California-based company Stemagen announced their findings Thursday in the journal Stem Cells. The scientists say they took the skin cells from two adult males and implanted the nuclei of those cells in human egg cells.

The scientists say they determined through genetic testing that at least three of the embryos were true clones. They say the genetic material in the other two embryos did not develop enough to draw conclusive tests.

Other scientists have greeted the news cautiously. South Korean scientist Hwang Woo-suk claimed in 2004 that he had used cloning technology to make human embryos, but he was later found to have faked parts of his research.

Human embryos are a source of stem cells that can be used to develop a number of different kinds of tissues. Doctors hope they could be used to treat diseases such as Parkinson's and Alzheimer's.

But opponents to the research, including President Bush, argue that destroying an embryo is tantamount to taking a human life. Last year, Mr. Bush vetoed legislation that would have eased restrictions on federally-funded medical research that uses stem cells from human embryos.

Some information for this report was provided by AP and Reuters.