Medical scientists working in South Korea claim to have made a significant breakthrough in the cloning of human cells for the treatment of diseases.

Writing in the journal Science, a team of Korean and US researchers says it has managed to extract stem cells from a cloned human embryo. The project is an important step toward growing patients' own replacement tissue to treat diseases.

The experiment is sure to revive controversy over human cloning, both in the US and internationally. The technique offers the potential of breakthrough treatments for diabetes, Parkinson's and other diseases, but any therapy is years away from being tested in people.

Scientists have used therapeutic cloning to partially cure laboratory mice with an immune system disease. And they know how to cull stem cells from human embryos left over in fertility clinics, offering the potential of cell therapy but not patient-specific treatment.

But attempts at cloning a human embryo in the stem cell quest have failed until now.

Dr Rudolf Jaenisch of the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts, said the technique is not of practical use at this point, stressing that years of additional research are required.