Following a new confirmed human case of bird flu in China's south-eastern Anhui province, the United Nations health agency is joining a mission by the Chinese Health Ministry to investigate exposure to the virus, which in a worst case scenario could mutate into a human pandemic that could kill tens of millions of people.

This is the third laboratory-confirmed case in China, and the second from Anhui. Two of the cases have been fatal. Since mid-October, China has reported a recurrence of poultry outbreaks in several parts of the country.

Ever since the first human case of bird flu, linked to widespread poultry outbreaks in Vietnam and Thailand, was reported in January last year, UN health officials have warned that the H5N1 virus could evolve into a global influenza pandemic if it mutates into a form which could transmit easily between people, states a UN release.

The so-called Spanish flu pandemic of 1918-1920 is estimated to have killed from 20 million to 40 million people worldwide. Overall, there have been more than 132 reported human H5N1 cases, 68 of them fatal, all in South-East Asia. Some 140 million domestic birds have died or been culled in an effort to curb the spread.

Vietnam has registered the largest number of human cases - 93 (42 fatal), followed by Thailand with 21 (13 fatal), Indonesia 11 (7 fatal), Cambodia 4 (4 fatal) and China 3 (2 fatal).

WHO official Dick Thompson said that the agency did not believe China was hiding any human cases of avian influenza. WHO believed China was notifying the organisation as rapidly as it could and was being as transparent as possible, he said.

UN health officials have said a human flu pandemic is inevitable.

"It is only a matter of time before an avian flu virus, most likely H5N1, acquires the ability to be transmitted from human to human, sparking the outbreak of human pandemic influenza. We don't know when this will happen. But we do know that it will happen," said WHO director, General Lee Jong-wook.