More than 1,000 migratory birds have died from avian flu in Qinghai province, western China, according to the country's agriculture ministry.
Emergency measures are in place to limit the disease's spread
The dead birds included bar-headed geese and great black-headed gulls, said Jia Youling, director of the ministry's Veterinary Bureau.
The death toll is much higher than previously reported, though officials said no humans had been infected.
The virus has killed at least 53 people in South East Asia since late 2003.
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
But no humans have been infected so far in China.
The World Health Organization 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.
China on Saturday ordered nationwide emergency measures to try to stop the spread of its latest outbreak, after 178 geese were found dead in Qinghai Lake.
The measures include banning people from habitats of migratory birds, immunising poultry raised near habitats and routes of migratory birds, and introducing quarantine measures in Qinghai.
China's previous outbreak of bird flu occurred last July, in the country's east.