The United Nations health body said it was "deeply concerned about the potential negative consequences" of work by two leading flu research teams who this month said they had found ways to make H5N1 into a easily transmissable form capable of causing lethal human pandemics.

The work by the teams, one in The Netherlands and one in the United States, has already prompted an unprecedented censorship call from U.S. security advisers who fear that publishing details of the research could give potential attackers the know-how to make a bioterror weapon.

The U.S. National Science Advisory Board for Biosecurity has asked two journals that want to publish the work to make only redacted versions of studies available, a request to which the journal editors and many leading scientists object.

In its first comment on the controversy, the WHO said: "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....has risks."

H5N1 bird flu is extremely deadly in people who are directly exposed to it from infected birds. Since the virus was first detected in 1997, about 600 people have contracted it and more than half of them have died.

But so far it has not yet naturally mutated into a form that can pass easily from person to person, although many scientists fear this kind of mutation is likely to happen at some point and will constitute a major health threat if it does.