The H5N1 virus responsible for the current virulent strain of bird flu has evolved into two genetically distinct strains, US scientists have confirmed.
Millions of birds have been culled to stymie bird flu
They fear this could increase the risk to humans - and complicate the search for an effective vaccine.
The US team analysed more than 300 H5N1 samples taken from infected birds and people between 2003 and summer 2005.
Details were presented to the International Conference on Emerging Infectious Diseases in Atlanta.
Prior to 2005 every known human case of bird flu had been caused by a particular subtype of the H5N1 virus, which infected people in Vietnam, Cambodia and Thailand.
But the latest analysis by the US Centers for Disease Control and Prevention identified a genetically distinct variant which appears to have emerged last year, infecting people in Indonesia.
Researcher Dr Rebecca Garten said: "As the virus continues its geographic expansion, it is also undergoing genetic diversity expansion
"Back in 2003 we only had one genetically distinct population of H5N1 with the potential to cause a human pandemic. Now we have two."
The H5N1 strain of bird flu has spread across Europe, Africa and parts of Asia and killed nearly 100 people worldwide and infected about 180 since it re-emerged in 2003.
Scientists fear it could evolve to gain the ability to jump easily from human to human, at which point it could trigger a pandemic, resulting in millions of deaths world-wide.
All influenza viruses mutate easily, and H5N1 appears to be no exception.
Dr Nancy Cox, chief of the CDC's influenza branch, stressed that neither of the two genetic subtypes of H5N1 had the ability to pass easily from human to human.
US authorities are now working on vaccines to combat both subtypes. However, the development of a definitive vaccine can only take place once the exact form of a pandemic virus is known.
Despite this researchers are confident that a vaccine that could protect against one subtype of H5N1 would also offer at least partial protection against the other.
Professor Hugh Pennington, a microbiologist at Aberdeen University, said flu viruses were expert at evolving rapidly to exploit new opportunities.
He said it was possible that either of the two subtypes could gain the ability to jump from person to person.
Science may have under-estimated the ability of H5N1 to spread across large areas of the world in the way that it has already done, he said.
"But no need to panic. The virus is still a bird virus, it is not yet a human virus, and it may never be a human virus.
"As long as we manage to keep it reasonably under control in the birds I think we can breathe relatively easily for at least a year or two."