Scientists in Hong Kong and the United States have identified a synthetic compound which appears to be able to stop the replication of influenza viruses, including the H5N1 bird flu virus.

The search for such new "inhibitors" has grown more urgent in recent years as drugs, like oseltamivir, have become largely ineffective against certain flu strains, like the H1N1 seasonal flu virus. Experts now question how well and how long the drug would stand up against the H5N1, should it unleash a pandemic.

Researchers in Hong Kong and the Unites States screened some 230,000 compounds that were catalogued with the U.S. National Cancer Institute, and found 20 that could potentially restrict the proliferation of the H5N1.

The experts told a news conference on Wednesday one of the compounds, compound 1 or NSC89853, showed promise.

"We have found a compound that is different from oseltamivir but which acts in the same way," said Leo Poon, a microbiologist at the University of Hong Kong.

"An analogy would be like we have a door with a keyhole, but the hole has changed, and the key, in this case oseltamivir, can't lock the door anymore," he told the news conference.

"But we have discovered another keyhole and another key which can lock the door."

Their finding was published in the latest issue of the Journal of Medicinal Chemistry.

In their experiment, the researchers infected separate batches of cultured human cells with seasonal flu virus and H5N1 and found that compound 1 prevented the replication of both types of viruses effectively.

"Given the problems with drug resistance, this compound can be used to develop a new drug," said Allan Lau, professor of pediatrics and adolescent medicine at the University of Hong Kong.

But he cautioned it would take as much as eight years for such a drug to be available on the market.

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 the U.S. Centers for Disease Control and Prevention said in March that 98 percent of all flu samples from the H1N1 strain were resistant to oseltamivir, which is manufactured by Roche AG and marketed under the brand Tamiflu.

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.