But they said it could take more than five years to convert their discovery of a potential weak point in a part of the virus that helps it to spread, into an effective drug.

The scientists, led by John Skehel of London's National Institute of Medical Research, said they found a cavity in the N1 or neuraminidase part of the H5N1 virus that could be exploited as a potential weak point.

"The hope is that any new information like this which shows something specific for the N1 neuraminidase will be able to be used to develop a drug against the H5N1 virus," Skehel said.

Mutations

World health officials are concerned the H5N1 strain of the bird flu virus could mutate into a form able to jump from human to human.

So far the 238 cases of human infection have been from direct contact with infected birds and nearly 60% of those infected have died.

However, scientists have said there is no evidence the virus is mutating towards making the leap between humans.
 
The best known drugs to tackle H5N1 infection in humans are oseltamivir, known as Tamiflu, and zanamivir, known as Relenza, both originally developed to fight other forms of human influenza.

And any new drug is still years away, Skehel said. "This is not just round the corner. It could easily be more than five years to develop a new drug."

Some countries have begun stockpiling Tamiflu and Relenza, but because the H5N1 virus mutates regularly it is not known how effective they would be against an H5N1 human pandemic.

An array of drugs - some specific to H5N1 and some like Tamiflu and Relenza to other N variants but effective against N1 - would not stop the virus mutating but would stop it developing drug resistance, he said.

"It is a race. You have got to try to keep ahead of variation - and in the case of H5N1 particularly the emergence of transmission from human to human," Skehel said.