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Tiny hairs in the ear which detect sounds have been regenerated to reverse deafness for the first time, say US researchers in the journal Neuron.

An injection of a drug led to in tests on mice.

Normal hearing was not restored, rather the mice went from hearing nothing to detecting sounds such as a door slamming or traffic.

Experts said it was "tremendously exciting" but warned treating humans was still a distant prospect.

To hear anything sound waves have to be converted into electrical signals which the brain will understand. The first step in the process takes place deep inside the inner ear where vibrations move tiny hairs and the movement creates an electrical signal.

Most hearing problems are as a result of damage to these hairs.

The study, by Massachusetts Eye and Ear and Harvard Medical School, looked at mice which were completely deaf and had virtually no hairs remaining in their ears.

A drug was used to target cells which normally support the individual hairs. It changed the destiny of the cells, by altering which genes were being used in the cells, to transform them into hair cells.

One of the researchers, Dr Albert Edge, said: "It hasn't been possible to regenerate hair cells in adult mammals before, this is very exciting. It shows for the first time that it's possible."

Brain scans showed that some sounds could be heard.

Dr Edge added: "There was a slight improvement, but not a huge improvement.

"They can detect a loud noise in a low frequency, something like a door slamming or traffic - but this is definitely not normal hearing."

Similar advances were made with stem cells in 2012. In that study, the connections between the hairs and the brain were broken and stem cells were used to create new nerves.

Rebuilding the hairs themselves is a far greater challenge.

Prof Dave Moore, the director of the Institute of Hearing Research in Nottingham, said the the workings of the ear were incredibly precise and constantly vibrating meaning rebuilding hairs was like trying to demolish and then rebuild a 15-storey building in the middle of a city, without damaging any of the surrounding buildings all in the middle of an earthquake.

He said: "It's a really promising development, but it is one which needs to be treated with considerable caution in terms of a human therapy.

"There's been a lot of false starts - hair cell regeneration was originally demonstrated in the 1980s and everyone thought it would just be a matter of years."

He said it was an exciting first step, but there was still a huge challenge ahead to develop a useable treatment.

Dr Ralph Holme, head of biomedical research at the charity Action on Hearing Loss, said: "The idea that a drug could be used to 'trick' the cochlea into producing new hair cells to improve hearing is tremendously exciting and offers real hope to the millions of people seeking a cure for their hearing loss.

"But, it is important to remember that this research is still at a very early stage and that only a partial recovery in hearing was observed.

"It will be important to test whether the approach will be useful in treating hearing loss that has been present for a long time."