The world's human death toll from bird flu has reached 103 since late 2003, the World Health Organization has said.
Millions of birds have been culled in an effort to curb the virus
The latest deaths from the H5N1 strain occurred in Azerbaijan, where five have died since February, the WHO reported.
The virus cannot pass easily from one person to another but there are fears it could mutate, triggering a pandemic.
US scientists have confirmed the H5N1 virus has evolved into two genetically distinct strains, potentially increasing the risk to humans.
The team from the US Centers for Disease Control and Prevention (CDC) also said this could complicate the search for an effective vaccine.
Four of those who died in Azerbaijan since February came from the Salyan region in the south-east of the country, while the fifth victim was in the Tarter region in the west, the WHO said.
Two other people from Azerbaijan have tested positive for bird flu during tests carried out at a British laboratory, the WHO said.
In other developments:
- Egypt reports a fourth suspected case of bird flu in humans
- Pakistan reports the country's first two cases of the H5N1 strain at two chicken farms
The team from the CDC said it had analysed more than 300 H5N1 samples taken from infected birds and people between 2003 and summer 2005.
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 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."
Scientists fear the virus 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.
CONFIRMED HUMAN DEATHS
Vietnam - 42
Indonesia - 22
Thailand - 14
China - 10
Azerbaijan - 5
Cambodia - 4
Turkey - 4
Iraq - 2
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.