The World Health Organization (WHO) said it was "deeply concerned" about research into whether the H5N1 flu virus could be made more transmissible between humans after mutant strains were produced in labs.
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.
Two top scientific journals said Tuesday they were mulling whether to publish full details on how Dutch scientists mutated the H5N1 flu virus in order for it to pass from one mammal to another.
Scientists fear H5N1 will mutate into a form readily transmissible between humans, with the potential to cause millions of deaths.
"The WHO takes note that studies undertaken by several institutions on whether changes in the H5N1 influenza virus can make it more transmissible between humans have raised concern about the possible risks and misuses associated with this research," The Geneva-based United Nations body said.
"WHO is also deeply concerned about the potential negative consequences.
"However, WHO also notes that studies conducted under appropriate conditions must continue to take place so that critical scientific knowledge needed to reduce the risks posed by the H5N1 virus continues to increase."
The WHO said research which could improve the understanding of such viruses was a scientific and public health imperative.
"While it is clear that conducting research to gain such knowledge must continue, it is also clear that certain research, and especially that which can generate more dangerous forms of the virus than those which already exist, has risks.
"Therefore such research should be done only after all important public health risks and benefits have been identified and reviewed, and it is certain that the necessary protections to minimize the potential for negative consequences are in place."
The H5N1 strain of avian influenza is fatal in 60 percent of human cases but only 350 people have so far died from the disease, largely because it cannot, yet, be transmitted between humans.
Indonesia has been the worst-hit country. Most human infections are the result of direct contact with infected birds.
In people it can cause fever, coughing, a sore throat, pneumonia, respiratory disease and, in about 60 percent of cases, death.
China is considered one of the nations most at risk of bird flu epidemics because it has the world's biggest poultry population and many chickens in rural areas are kept close to humans.
A man is in critical condition after testing positive for the H5N1 virus in the southern Chinese city of Shenzhen, state media said Saturday.