The UN body added the outbreaks that had killed 18 people in Southeast Asia had not become an epidemic.

The H5N1 influenza has forced the slaughter of millions of birds across Asia and has alarmed health experts because it is deadly to humans.

But it has not yet acquired the mutations it needs to pass easily among people, the WHO said.

"WHO has today received the results from studies of two viruses taken from members of a family cluster of H5N1 infection in Vietnam," the WHO said in a statement posted on its website at www.who.int.

"Virus genetic materials from two fatal cases in this cluster - sisters aged 23 and 30 years - have now been fully sequenced by the Government Virus Unit of Hong Kong's Department of Health. Both viruses are of avian origin and contain no human influenza genes," the WHO said.

"This finding, which indicates that the virus has not changed to a form easily transmitted from one person to another, is consistent with earlier findings from epidemiological investigations.

"No illness has been reported in other family members, in the local community, or in health workers involved in care of these patients," it added.

Deadly potential

The H5N1 influenza has hit Vietnam the hardest, killing 13 people. It has also infected flocks in Korea, Japan, Thailand, Cambodia, China, Laos and Indonesia.

Fifty million birds have been
culled across Asia

Health experts have warned the virus has the potential to cause a serious epidemic among people if it acquires the ability to pass easily from person to person.

It now remains mostly a bird virus, but when it does infect people it makes them seriously ill and usually kills.

Most influenza strains originate in birds and experts believe influenza viruses swap parts with one another in the body.

So if a person infected with human influenza also becomes infected with the new H5N1 virus, it is possible the H5N1 could take on the genes that allow human flu to easily infect people.

Researchers reported this week that genetic tests of a flu virus that killed 40 million people worldwide in 1918 showed it had not mutated very much from an original birdlike form - suggesting very little mutation was needed to turn a bird virus into a human pathogen.

Governments in affected countries are ordering the mass slaughter of chickens and other birds to try to control the virus.

Experts say such a slaughter may have stopped an outbreak of H5N1 in Hong Kong in 1997.

Fifty million birds have been culled in Asia so far.