Shigeru Omi, the WHO's regional director for the Western Pacific, said on Thursday that Asia was still the epicentre of the threat to global health.
"As the new cases of human infection with the H5N1 virus in Turkey show, the situation is worsening with each passing month and the threat of an influenza pandemic is continuing to grow every day," he told a two-day meeting of Asian countries and international organisations on bird flu in Tokyo.
But he also said that a pandemic was not inevitable if countries and health bodies responded quickly.
Experts say the H5N1 virus could become more active in the colder months in affected regions, and spread in east Asia as people slaughter chickens for Lunar New Year celebrations.
The more it becomes entrenched in poultry flocks, the greater the risk that more humans will become infected.
So far, the virus is reported to have infected about 150 people, killing 78 of them in six countries.
Indonesia said on Thursday that local tests established the cause of death of a 29-year-old woman as bird flu.
While it remains essentially a disease in birds, scientists fear the virus could mutate into a form that could spread easily between humans, causing a pandemic in which millions could die.
While there is no evidence of human-to-human transmission in Turkey, the large and rapid rise in the number of cases has worried experts.
The H5N1 virus has been found in wild birds and poultry in a third of Turkish provinces.
It has killed at least two children and infected more than a dozen people in little more than a week.
David Nabarro, the UN's senior co-ordinator for avian influenza, said on Wednesday: "The pace of fatalities appears to have fallen off quickly. But it is as yet unclear whether this is because the virus has modified or Turkey's approach has been successful."
He also said the virus appeared to be spreading via wild birds, "but once in an area, it spreads locally", leading to occasional human infections.
Iranian newspapers reported on Thursday that all poultry within 15km of Iran's 486km border with Turkey are to be destroyed to create a buffer zone against bird flu transmission.
Reports said that more than 55,000 birds would be killed and farmers were expected to receive compensation.