An outbreak of bird flu in northwest China is more lethal than previously thought, UN health experts have warned.
Infected birds could introduce the virus along their migration routes
They said 5,000 migratory birds had died in the outbreak, in Qinghai province, five times China's estimate.
"The virus has obviously changed to be more pathogenic to animals. What it means to humans we don't know," a World Health Organization official said.
The virus has killed at least 53 people in South East Asia since late 2003, though none of them were in China.
WHO China representative Henk Bekedam said the H5N1 outbreak at Lake Qinghai was particularly worrying because it affected migratory birds, including gulls, geese, shelducks and cormorants.
"The biggest concern I would have is that we have many birds that are asymptomatic and they can fly from here to 1,000km (621 miles) further up. That's troublesome," he said, the AFP news agency reported.
The organisation said it was vital that China carry out more tests on the birds before they begin migrating to other parts of the region.
H5N1 BIRD FLU VIRUS
Principally an avian disease, first seen in humans in Hong Kong, 1997
Almost all human cases thought to be contracted from birds
Isolated cases of human-to-human transmission in Hong Kong and Vietnam, but none confirmed
Chinese authorities have cordoned off a 50km (31 mile) radius around Lake Qinghai and nature reserve. However, the WHO says only 12 birds and two people have been tested for the strain so far.
China has not carried out a cull of the birds, a common practice with infected domesticated flocks, because they are rare and protected species, the WHO said.
Fears about the H5N1 virus have led to the slaughter of millions of chickens and ducks across Asia in the last 18 months.
The WHO is alarmed that the more the virus spreads, the greater the chances it could combine with the human flu virus and trigger a flu pandemic.