International scientists on Friday agreed to a temporary two-month halt to controversial research on a bird flu virus that may be easily passed among humans, citing global health concerns.

Two separate teams of researchers, one in the Netherlands and the other in the United States, found ways late last year to engineer the H5N1 virus so that it was transmitted among mammals, something that has previously been rare.

The breakthrough raised alarm the method could fall into the wrong hands and unleash a massive flu pandemic that could cost millions of lives, and a US advisory panel in December urged that key details should remain unpublished.

"Resulting from concerns about recent research on avian influenza, scientists working on transmission of the H5N1 strain have agreed to halt this area of research for 60 days to allow time for international discussion," said the letter from the researchers, published by the journals Science and Nature.

"We recognize that we and the rest of the scientific community need to clearly explain the benefits of this important research and the measures taken to minimize its possible risks," it added.

"We propose to do so in an international forum in which the scientific community comes together to discuss and debate these issues."

The US journal Science and the British magazine Nature have been working with researchers on edits to the manuscripts but have not said when or if they would eventually be published.

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.

Shortly after the research came to light, the US National Science Advisory Board for Biosecurity, a non-governmental panel of 23 experts, urged the two leading scientific journals to edit out key details.

National Institutes of Health experts have been working with researchers and editors to pare back the reports for publication, despite outcry from some scientists who said the work must be shared.

Among them was top Dutch scientist Ron Fouchier, who downplayed the threat to biosecurity and told AFP in December that the data "needs to be shared with countries where H5N1 viruses cause outbreaks so that the countries can now be on the lookout if these mutations arise."

However, Fouchier and other top researchers all signed the latest letter agreeing to a temporary halt to research.

"We would like to assure the public that these experiments have been conducted with appropriate regulatory oversight in secure containment facilities by highly trained and responsible personnel to minimize any risk of accidental release," the letter said.

The research aimed to provide "positive public health benefits," it added, but pointed out that whether or not these ferret-adapted flu viruses "have the ability to transmit from human to human cannot be tested."

NIH director Francis Collins and Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases issued a joint statement in support of the moratorium.

"We applaud the decision by these scientists, who have demonstrated great responsibility and flexibility in pausing their work to allow for a full dialogue about the risks and benefits of this research," it said.

"Officials with the World Health Organization are now working to organize a forum for the international scientific community to discuss these issues in the coming weeks."

While some experts questioned whether 60 days would be enough to sort out the complex issues surrounding the research, researcher Adolfo Garcia-Sastre of the Mount Sinai School of Medicine said he hoped it would be "ample time."

"After that I hope that we agree on the best way how to quickly advance scientific progress while minimizing potential risks," he told AFP.