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Early results from the world's first human trial using embryonic stem cells to treat diseases of the eye suggest the method is safe, say researchers.

US firm Advanced Cell Technology told The Lancet how two patients who had received the retinal implants were doing well, four months on.

Trials of the same technique have now started at London's Moorfields Eye Hospital.

But experts say it will be years before these treatments are proven.

The aim of these first human studies is to establish that the treatment is safe to use.

The treatment takes healthy immature cells from a human embryo, which are then manipulated to grow into the cells that line the back of the eye - the retina.

Experts hope that by injecting these cells into a diseased eye, they will be able to restore vision for people with currently incurable conditions such as Stargardt's disease - one of the main causes of blindness in young people.

Advanced Cell Technology, along with the Jules Stein Eye Institute at the University of California, Los Angeles, are reporting their first experiences with this treatment in human trials.

The study involved one elderly patient in her 70s with dry age-related macular degeneration - the leading cause of blindness in the developed world - and another female patient in her 50s with Stargardt's disease.

Both had very poor vision and were registered blind.

Each patient was given an injection containing 50,000 of the retinal pigment epithelium cells into one of their diseased eyes.

After surgery, structural evidence confirmed the cells had attached to the eye's membrane as hoped, and continued to survive throughout the next 16 weeks of the study.

Furthermore, the procedure appeared to be safe, causing no signs of rejection or abnormal cell growth.

Although this study is not designed to see if the procedure actually works, the researchers say their results do suggest that their patients' vision has improved slightly.

But they say it is still too soon to make any firm conclusions and that many more years of investigation will be needed to confirm that the treatment is both safe and effective.

They told The Lancet: "The ultimate therapeutic goal will be to treat patients earlier in the disease processes, potentially increasing the likelihood of photoreceptor and central visual rescue."

But even if this does become possible, such treatments would face stiff opposition by critics who say it is ethically wrong to use human embryonic tissue.

Dr Dusko Ilic, Senior Lecturer in Stem Cell Science at Kings College London, said that these early findings did not necessarily hint towards a viable treatment.

"We should keep in mind that people are not rats.

"The number one priority of initial clinical trial is always patient safety. If everyone expects that the blind patients will see after being treated with human embryonic stem cell-derived retinal pigment epithelium, even if the treatment ends up being safe (which is what Advanced Cell Technology are trying to determine in this trial), they risk being unnecessarily disappointed."

UK stem cell expert Chris Mason added: "We do not have a complete answer yet. But it is a valuable next step."