Addressing the UN agency's 34-state executive board, WHO Director-General Margaret Chan said that in the past three years the H5N1 bird flu virus had proven virulent.
"As long as the virus continues to circulate in birds, the threat of a pandemic will persist. The world is years away from control in the agricultural sector," she said.
Since the disease re-emerged in 2003, there have been 267 infections in humans, mostly in southeast Asia, and 161 deaths. Nearly half the fatalities occurred in 2006 alone, Chan said.
Although the disease remained primarily an avarian disease, it had lost none of its virulence when it did jump to humans, with the death rate in 2006 touching 70% compared with 60 % over the three years.
The WHO has long warned that the virus, which first erupted in 1997 in Hong Kong, could trigger a global pandemic if it mutates into one capable of being passed on easily between humans. So far virtually all human cases have involved close contact with infected birds.
Chan, who took over as head of the Geneva-based agency earlier this month, said that it was impossible to predict when, if at all, such a mutation could take place.
"Influenza viruses are notoriously sloppy, unstable and capricious. It is impossible to predict their behaviour," she told the board, which meets twice a year.