WHO stressed the virus was not yet spreading easily among people -- a necessary first step toward a feared pandemic.

"To date, the investigation has found no evidence of spread within the general community and no evidence that efficient human-to-human transmission has occurred," the WHO said.

And samples from the victims have not shown any big genetic changes.

"Sequencing of all eight gene segments found no evidence of genetic reassortment with human or pig influenza viruses and no evidence of significant mutations," the WHO statement read.

"The human viruses from this cluster are genetically similar to viruses isolated from poultry in North Sumatra during a previous outbreak."

Several health experts have expressed concern about how long it has taken to get information about the outbreak in Sumatra and said it shows that if H5N1 did evolve into a pandemic form, there would be little chance of stopping it.

"It does shake your confidence in the fire blanket approach," Dr. Eric Toner of the Center for Biosecurity at the University of Pittsburgh Medical Center said in an interview.

A 'fire blanket' approach would involve using drugs, isolation and quarantine to stop an early outbreak of human-to-human spread of the virus.

WHO is trying to encourage countries to report suspected bird flu cases quickly. But there are fears that some countries may try to cover up cases, in order to deal with them quietly.

WHO and other agencies have been on alert after an Iranian medical official told Reuters on Monday that a 41-year-old man and his 26-year-old sister from the northwestern city of Kermanshah had tested positive for bird flu.

Health Minister Kamran Lankarani denied this although international health officials are still investigating.

"Fortunately, these two cases were negative for avian flu. There is no confirmed case until now," Lankarani told Reuters at the WHO annual meeting in Geneva.

The two siblings were among five members of a family who became sick and the other three remain in the hospital.

A Japanese currency trader said the WHO report triggered some selling of yen in early Asian trading, which weighed on Taiwan's currency. The Australian dollar was also hurt by the report, and Australian bond futures reached four-week highs.

Markets are sensitive to bird flu reports as a pandemic would probably hurt consumption across the globe, slowing growth and putting downward pressure on interest rates.