A scientist researching a potentially highly lethal airborne version of the H5N1 bird flu virus said on Wednesday he must be allowed to pursue his studies if deadly pandemics are to be prevented.

Despite declaring last week a 60-day moratorium on the studies to allay security fears, Yoshihiro Kawaoka argued in a commentary in the journal Nature it was urgent and vital that his work continue.

Kawaoka, of Tokyo University and the University of Wisconsin-Madison in the United States, is a lead researcher on one of two recent studies showing how H5N1 can be transmitted through airborne droplets, and his work is at the centre of an international row over whether its findings should be censored.

In December a U.S. advisory board asked two leading journals, Nature and Science, to withhold details of both studies for fear it could be used by bioterrorists. The journals have accepted the studies but have not yet said if they will publish them in full.

Last week, the two teams - Kawaoka's and a second team led by Ron Fouchier at Erasmus Medical College in the Netherlands - said they would temporarily suspend their research because of the concerns.

But writing in Nature on Wednesday, Kawaoka argued it would be "irresponsible" and dangerous not to continue researching highly pathogenic bird flu viruses.

Flu viruses constantly mutate and can cause pandemics.

Kawaoka said some elements of worrying mutations that both teams had predicted were possible had already been detected in H5N1 viruses circulating naturally in certain countries.

"Some people have argued that the risks of such studies - misuse and accidental release, for example - outweigh the benefits," he wrote.

"I counter that H5N1 viruses circulating in nature already pose a threat, because influenza viruses mutate constantly and can cause pandemics with great losses of life."

He cited the 1918-19 Spanish flu outbreak that killed an estimated 20 million to 50 million people worldwide.

"We cannot give up"

First detected in 1997 in Hong Kong, H5N1 has devastated duck and chicken flocks in Cambodia, China, Egypt, India, Indonesia and Iran, and has reached the Middle East and Europe through wild birds.

Lab analysis has confirmed that 578 people have been infected since 2003. Of those, 340 have died - a death rate never before seen from a flu virus.

By comparison, the Spanish flu killed an estimated 0.5 percent of those infected, while seasonal flu in the United States kills about 0.003 percent of those who catch it.

People can contract H5N1 in its current form only through close contact with ducks, chickens or other birds that carry it, and not from infected people.

Kawaoka's and Fouchier's teams have found that with just three induced mutations the virus can become transmissible through air between ferrets, which are considered good models of how flu viruses behave in people.

It is not known whether the mutant H5N1 can be spread between people in a similar way, but the fear is that it might, unleashing a highly lethal pandemic.

Kawaoka argued it "would be irresponsible not to study the underlying mechanisms" of how these mutations might come about, and said this research had implications the world's ability to be prepare itself for another flu pandemic.

"It is imperative that these viruses are monitored closely so that eradication efforts and counter-measures...can be focused on them," he wrote.

He argued that the U.S. National Science Advisory Board for Biosecurity's request for publication of the studies to be censored would not eliminate the possibility of experiments being replicated by people bent on doing harm.

In reality, he said, there is already enough information out there to allow someone to make a transmissible virus.

Kawaoka proposed that instead of halting or censoring research, the international community should convene to discuss how to minimize risk while also supporting scientific discovery.

"Flu investigators (including me) have agreed to a 60-day moratorium on avian flu transmission research because of the current controversy," he said. "But our work remains urgent - we cannot give up."