* Scientists halted research voluntarily a year ago
* US raised concerns virus could be used by terrorists
* Benefits of studies outweigh risks, scientists say (Adds details, quotes from Dutch and Japanese researchers)
By Kate Kelland
LONDON, Jan 23 (Reuters) - Scientists around the world declared an end on Wednesday to a moratorium on researching mutant forms of the deadly H5N1 bird flu that had raised international biosecurity concerns.
Announcing their decision to resume what they say are risky but essential studies of the avian flu strain, the scientists said the work would only be carried out in the most secure sites in countries that agree it can go ahead.
That will allow work to start again in key laboratories in the Netherlands and elsewhere but not yet in the United States or U.S.-funded research centres, pending further safety and security guidelines there.
Scientists voluntarily halted research on H5N1 transmission a year ago due to fears that information about how to create potentially dangerous viruses could be used for bioterrorism.
Flu experts said they have recognised those fears and worked hard to calm them, and now it is time to push on. They say the studies are essential for a deeper understanding of H5N1, which many fear could one day spark a lethal pandemic in humans.
"We want the world to be better prepared than we currently are when an H5N1 virus causes a pandemic," said Yoshihiro Kawaoka of Tokyo University, a leading researcher on avian flu.
"We understand the risks associated with our research and we take every precaution to conduct H5N1 virus experiments safely."
He told reporters on a teleconference the research would boost efforts to develop global flu "biosurveillance", early warning systems, as well as better flu drugs and vaccines.
In a letter published jointly by the journals Nature and Science, 40 flu researchers from the United States, China, Japan, Britain, the Netherlands, Canada, Hong Kong, Italy and Germany wrote: "This research - as with any work on infectious agents - is not without risks.
"However, because the risk exists in nature that an H5N1 virus capable of transmission in mammals may emerge, the benefits of this work outweigh the risks."
Wendy Barclay, a flu virologist at Imperial College London and one of the letter's signatories, said lifting the moratorium would lead to scientific discoveries that would have "direct consequence for human and animal health".
All research into H5N1 transmission was halted in January 2012 after teams at the University of Wisconsin in the United States and at the Dutch Erasmus Medical Centre in Rotterdam created mutant forms that can be transmitted directly among mammals, meaning they could in theory also pass between people.
MUTATIONS HAPPEN IN NATURE
Currently, bird flu can be transmitted from birds to birds, and birds to humans, but not from humans to humans. When it does pass from birds to humans, it is usually fatal. Scientists are concerned the same mutations needed to make it transmissible among mammals in a lab could one day happen in nature.
When news of the work emerged late in 2011, it prompted the U.S. National Science Advisory Board for Biosecurity to call for the scientific papers about it to be censored to prevent details falling into the wrong hands.
The censorship call sparked a fierce debate about how far scientists should be allowed to go in manipulating infectious agents in the name of research.
Barclay said this had been a "knee-jerk response from certain quarters previously naive of this approach, expressing horror that scientists were brewing up deadly diseases."
During the moratorium, the World Health Organisation (WHO)recommended that scientists explain the biological and other security measures they use to contain the virus and make more effort to show why the research is so important.
"The laboratories have expanded on their containment and security system ... and I think the value of the results has been recognised," John McCauley, director of the WHO collaborating centre for flu research at Britain's National Institute for Medical Research said.
Ron Fouchier, from the Rotterdam lab which led one of the studies, said his team would start fresh research on H5N1 viruses "in the next few weeks".
"We really need to understand how these viruses become airborne," he told reporters in the teleconference, saying that was the primary goal of the work. (Reporting by Kate Kelland; Editing by Robin Pomeroy)