The World Health Organization (WHO) says it is "deeply concerned" about researchers creating a more contagious and fatal form of the H5N1 bird flu.

Researchers in the Netherlands have manipulated the virus to make it more transmissible among humans, and it could potentially kill millions if released into the public.

The findings were set to be released in the U.S. journal Science, but the National Science Advisory Board for Biosecurity, an independent committee that advises the U.S. Department of Health and Human Services and other federal agencies, reviewed it last Tuesday and warned that bioterrorists could replicate the study methods to create a weapon of biological warfare.

On Friday, the WHO echoed the agencies' sentiments by saying that the studies could open the door to "possible risks and misuses."

The current H5N1 strain is more often found in birds and not easily transmissible to human.  Hundreds of millions of birds have died from the virus since it was first identified in 1996. So far an estimated 600 people have been infected. About 60 percent of those who get the bird flu die, according to the WHO.

Ron Fouchier of Erasmus Medical Center in Rotterdam, Netherlands, said he created the contagious form of the deadly H5N1 bird flu strain "easily" by mutating a few genes within the strain. He agreed to exclude methodology details from his published reports on the new strain.

"We know which mutation to watch for in the case of an outbreak, and we can then stop the outbreak before it is too late," Fouchier said in a statement on the medical center's website. "Furthermore, the finding will help in the timely development of vaccinations and medication."

Fouchier is one of many researchers worldwide looking at what kind of mutations would make the H5N1 more dangerous to humans.

In May, the WHO member countries adopted a Pandemic Influenza Preparedness Framework which set rules on sharing information about flu viruses that have pandemic potential.

"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," the WHO said in a public statement.