The United Nations is warning an outbreak of avian flu in China is more lethal than first thought.

A total of 5,000 birds have died on an island in northwest China´s Qinghai province, according to officials from the World Health Organisation (WHO) and the Food and Agriculture Organization (FAO).

The experts from WHO and FAO have just returned from a visit to the area and say previous reports had indicated only 1,000 migratory birds had died on the island.

"This is the first time we磛e seen large numbers of migratory birds dying from bird flu," said WHO spokesperson Julie Hall.

"So the virus has obviously changed to be more pathogenic to animals. What it means to humans we don磘 know," she told reporters in Beijing.

There are 189 species of birds on the island, but so far Chinese officials have only tested five, the WHO and FAO experts said.

The migratory birds are still on the island, but they will begin flying off to other parts of China and to neighboring countries in about a month.

"So we believe there is a window of opportunity to study these birds... but this window is only a month and a half," Hall said.

She says it´s vital to test the birds before they leave, to provide an adequate warning to other countries.

June 28 (Bloomberg) -- An outbreak of bird flu in western China´s Qinghai province may spread to other provinces and possibly other countries, carried by migratory birds that have the virus, two international health teams said in Beijing.

Health workers have a ``narrow window of opportunity创 to determine whether birds not killed by the virus in Qinghai, where bird flu has killed 5,000 migratory geese and other birds since early May, are carriers capable of infecting species in other areas, said Julie Hall, Communicable Disease Surveillance and Response coordinator with the World Health Organization.

``We have no idea whether the other 184 species in the area are asymptomatic carriers,创 Hall said in a joint press briefing with the Food and Agricultural Organization. ``There is limited tagging and understanding of where they migrate to,创 when they begin moving in August and September.

The WHO is concerned that H5N1, a virus strain that killed migratory birds in Qinghai, may mutate into a strain easily transmissible between humans, and is urging China to sample other species in the area.

The Geneva-based arm of the United Nations also wants China to share the gene sequence for the virus that killed the birds in Qinghai to determine whether it´s different or more threatening to humans than strains found elsewhere in the region.

Last week, teams from WHO and FAO arrived in Qinghai to inspect the site of an outbreak of avian influenza and see how prepared remote parts of China are in dealing with an outbreak.

Japanese Poultry

``We see avian flu to be an international issue that requires international cooperation,创 said Liu Jianchao, spokesman of China´s Foreign Ministry. ``We are willing to cooperate with the WHO and international community on fighting epidemics.创

China reported this month that 2,177 geese may have the virus in Tacheng city in neighboring Xinjiang province. A similar outbreak among migratory birds in neighboring Qinghai province last month killed 1,000 migratory birds, Jia Youling, director- general of the veterinary bureau of the Ministry of Agriculture in Beijing, said on May 27.

Japanese authorities ordered the slaughter of all 25,000 chickens at a poultry farm north of Tokyo after a weak strain of the H5N1 virus, called H5N2, was identified as the cause of deaths among the chickens that started in April, the Asahi Shimbun reported. The cull at the farm in Mitsukaido, Ibaraki Prefecture, began yesterday and will be completed this week, the paper said.

The WHO has called on governments to prepare for an influenza pandemic, similar to those that killed between 20 million and 40 million people worldwide in 1918-1919, about 1 million in 1957- 1958 and between 1 million and 4 million in 1968-69. Scientists fear populations will have no immunity to a mutated strain of avian influenza.

The H5N1 strain first emerged in Hong Kong and China eight years ago and has killed 38 people in Vietnam, 12 in Thailand and four in Cambodia, the WHO reported June 17.