WASHINGTON, July 6 (Reuters) - The spread of avian flu virus among migrating geese and other birds at a wildlife refuge in China means the birds could carry the devastating virus out of Asia, scientists reported on Wednesday.

This makes avian flu even more of a global threat than it already is, the scientists said in reports published jointly by the journals Science and Nature. Health officials fear avian influenza could cause a pandemic of human disease.

At least 1,000 dead birds have been found at Lake Qinghaihu, a protected nature reserve in western China, according to two separate reports. United Nations scientists said last week the number had topped 5,000.

"The occurrence of highly pathogenic H5N1 avian influenza virus infection in migrant waterfowl indicates that this virus has the potential to be a global threat," Jinhua Liu of China Agricultural University, George Gao of the Chinese Academy of Sciences and colleagues wrote in their report in Science.

"Lake Qinghaihu is a breeding center for migrant birds that congregate from Southeast Asia, Siberia, Australia and New Zealand."

The latest outbreak of the virus that started in 2003 has killed 39 people in Vietnam, 12 in Thailand and four in Cambodia. The World Health Organization has said the virus would kill millions of people worldwide if it acquires the ability to pass easily from human to human.

So far it has not, but influenza is extremely mutation-prone.

The virus, which affects ducks with little harm but which kills chickens, had not before been seen to transmit among wild birds.

KEY BREEDING GROUND

"This lake is one of the most important breeding locations for migratory birds that overwinter in Southeast Asia, Tibet and India," Gao´s team wrote.

"Several species were infected, including the bar-headed goose (Anser indicus), great black-headed gull (Larus ichthyaetus) and brownheaded gull (Larus brunnicephalus)."

One of the symptoms seen in the wild birds was diarrhea, which could mean the virus would spread in contaminated water.

Yi Guan of the University of Hong Kong and colleagues did a genetic analysis of the virus taken from the dead birds and found it is closely related to the strain that has caused human illness in Thailand and Vietnam.

But the sequences appeared to have mutated slightly, they added.

"This outbreak may help to spread the virus over and beyond the Himalayas and has important implications for developing control strategies," they wrote in their report, published in Nature.

It spread quickly, causing paralysis and staggering, they said.

"By 4 May, bird mortality was more than 100 a day; by 20 May, the outbreak had spread to other islets, with some 1,500 birds dead."

Their genetic analysis suggested the virus was introduced just one time to the lake, meaning a single infection could have spread quickly.

The outbreak could burn itself out, but the large migratory bird population at the lake made this unlikely, they wrote.

"The viruses might also move to other migratory species that could act as carriers, remaining highly pathogenic for domestic chickens and possibly humans."

United Nations officials said on Tuesday that bird flu is entrenched in Asia and predicted it would take up to a decade to rid the region of the virus.

At a meeting in Kuala Lumpur, they said more than $100 million would be spent over the next three years on improving the detection and reporting of outbreaks, and in combating the virus.