Christina Ficara - All Headline News Staff Reporter

The warning follows the deaths of 5,000 migratory birds in the outbreak, in Qinghai province; which is five times greater than China's official estimate.

Despite the obvious change in severity to the animals, health officials are still unsure of the threat it poses to humans.

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, says the H5N1 outbreak at Lake Qinghai was most threatening because it affected migratory birds, including gulls, geese, shelducks and cormorants.

Bekedam told local media, "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."

WHO reported it was vital that China carry out more tests on the birds before they begin migrating to other parts of the region.

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.

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.