Top US scientists on Wednesday defended their bid to stop details of a mutant bird flu virus from being published and called for global cooperation to ward off an uncontrollable pandemic.

Meanwhile, scientists involved in the experiments said they are cooperating with government officials and the editors of the journals Science and Nature to pare down their research for publication in the coming weeks.

The controversy arose when two separate research teams -- one in the Netherlands and the other in the United States -- separately found ways to alter the H5N1 avian influenza so it could pass easily between mammals.

Until now, bird flu has been rare in humans, but particularly fatal in those who do get sick. H5N1 first infected humans in 1997 and has killed more than one in every two people that it infected, for a total of 350 deaths.

The concern is the virus could mutate and mimic past pandemic flu outbreaks such as the "Spanish flu" of 1918-1919 which killed 50 million people, and outbreaks in 1957 and 1968 that killed three million.

The recommendations from a non-governmental advisory panel that key details of the newly altered virus be withheld drew fire from some scientists who saw it as censorship of material that is essential for surveillance and the hunt for vaccines.

Some experts have pointed out that the data does not show whether the engineered virus is actually transmissible between humans, and a major risk is that the virus would emerge on its own in nature.

But the chair of the National Science Advisory Board for Biosecurity, Paul Keim, told AFP that even though the research has already been discussed by Dutch scientists at public forums, all 23 members voted unanimously to urge editors to withhold the data in case it fell into the wrong hands.

"We were very worried about the perception -- that the world would view this as the US holding back information that is important for basic research and public health," Keim told AFP.

"The US government is the one that paid for these experiments, so I think that it is the responsibility of the US government to step forward at this time. But it needs to be a global effort and we need a global consensus," he said.

"This is such a dangerous biological weapon, it would not be controllable. Whoever used it would doubtlessly decimate their own people as well," added Keim.

That scenario is not far-fetched, given the existence of radical elements throughout the world, from doomsday cults to suicide bombers, and even leading world governments, he said.

"The United States has participated in a strategy of mutually assured destruction in the area of nuclear weapons for 60 years now," he said.

"So to say that people wouldn't construct and use a weapon that is so deadly that it couldn't be controlled even by the most developed countries is, I think, a fallacy."

A spokeswoman for the journal Science said talks are under way to get the detailed information to scientists who need it through established channels, such as the World Health Organization.

Arranging for access has been a key argument of the researchers involved. Both teams have agreed to "respect" the NSABB recommendations, but have also expressed some reluctance to water down their findings.

"It is harder when you talk about removing information from a manuscript," said Terry Devitt, a spokesman for the University of Wisconsin where one of the studies was based.

Ron Fouchier, whose team at the Rotterdam Erasmus Medical Centre announced the mutant version in September, told AFP the threat was overblown.

"Recreating this virus is definitely not easy. You need highly skilled people and a very large team, as well as specialized facilities to do this type of work," he said.

Anthony Fauci, head of the US National Institute of Allergy and Infectious Diseases, said the move did not amount to censorship and vowed that any "legitimate" researchers would be able to seek the full details for their own study.

"If their credentials are appropriate they will have access to that information," he told AFP.

According to Bruce Hirsch, a physician and infectious diseases expert at North Shore University Hospital in Manhasset, New York, the trade-off is worthwhile.

"To accept a level of censorship in the medical journals, which is extraordinary, I think is appropriate given the complexities of the world that we live in," said Hirsch, who was not involved in either study.

Keim said perhaps the researchers should have given more thought to the potential dangers of their experiments early on.

"In the future we need to be aware of the consequences of the research sooner in the process rather than just when it gets to the point of publication," he said.

"These experiments were done with the best intentions, but the negative consequences are so great."