Experts on Friday urged governments on Friday to diversify their stockpiles of drugs and called for more new medicines to fight what could be the world's next flu pandemic caused by the H5N1 bird flu virus.

Many advanced countries stock up on oseltamivir and zanamivir, two varieties of the same class of drugs that stops the H5N1 virus from multiplying.

But oseltamivir has proven to be largely useless in fighting the H1N1 seasonal human influenza virus and experts are questioning how well, and how long, the drug would stand up against the H5N1 virus, should it unleash a pandemic.

"We have been extremely foolish on our policies of stockpiling drugs. We have been stockpiling two varieties of the same drug," virologist Robert Webster at the St Jude Children's Hospital in the United States said at a medical conference in Hong Kong.

He said the resistance of the H1N1 virus to oseltamivir was as high as 98 percent worldwide.

"The likely scenario is that the (H5N1) virus will become resistant when you start using more and more (of one) drug, you get resistant (H5N1) mutants," he told Reuters later.

The U.S. Centers for Disease Control and Prevention said in December 2008 that 49 out of 50 H1N1 virus samples tested were no longer sensitive to oseltamivir, which is made by Roche AG and Gilead Sciences Inc..

Relenza, known generically as zanamivir, is made by GlaxoSmithKline under license from Australia's Biota Inc.

Viruses and bacteria are sturdy organisms that fight hard to survive and adapt swiftly to drugs that are used to kill them, quickly becoming resistant to them.

Experts at the conference said most H1N1 viruses were sensitive to oseltamivir just a few years ago but learnt to adapt to the drug very quickly.

They warned that H5N1 could learn to adapt quickly to oseltamivir, just as H1N1 has done, because both viruses share a similar "N1" protein component.

Scientists in Vietnam reported in the past how certain strains of H5N1 were no longer sensitive to oseltamivir, resulting in the deaths of several infected patients.

Experience with viruses similar to H5N1 suggests that it would best be tackled with a combination of drugs, Webster said.

"Studies with HIV, leukemia have shown that we have to use multiple drugs," he said. He suggested that oseltamivir and zanamivir be trialled alongside other drugs like ribavirin and the adamantane class of drugs, like amantadine and rimantadine.

Malik Peiris a microbiologist with the University of Hong Kong said: "There is a need for developing new antivirals (drugs). Some are in very early clinical trials but development should be stepped up so there is a diversification of options."